The Lower Devonian Upper Gasp6 Limestones in eastern Gasp6: …uregina.ca/~chiguox/s/2001...

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 49, NO. 2 (JUNE, 2001), P. 346-365

The Lower Devonian Upper Gasp6 Limestones in eastern Gasp6: carbonate diagenesis and reservoir potential

DENIS LAVOIE, GUOXIANG CHI Geological Survey of Canada (Quebec)

880 Chemin Sainte-Foy, C.P. 7500 Sainte-Foy, QC

T2L 2A7

MARTIN G. FOWLER Geological Survey of Canada (Calgary)

3303 - 33rd Street NW

Calgary, AB T2L 2A7

ABSTRACT

The Lower Devonian Upper Gasp6 Limestones consist of the Forillon, Shiphead and Indian Cove formations. The depositional basin and stratigraphic succession were controlled by synsedimentary extension faults. Three major paleogeographic domains are recognized: a northern proximal outer shelf, a central distal outer shelf, and a southern toe- of-slope. Numerous oil seeps occur in fractured limestones of the central domain, and limited hydrocarbon production has been recorded from these units.

Primary porosity occurs in the northern domain in the Indian Cove Formation brachiopod-rich facies. Local meteoric dissolution enhancement of porosity is the result of late influx of meteoric waters, as inferred from the nature of fracture-filling calcite cements. Porosity enhancement is restricted to the northern domain, but this area is barren of hydrocarbons.

Multiple episodes of burial fracturing, dissolution, and calcite cementation are found in the central and southern domains. Petrographic evidence indicates three distinct diagenetic events occurred before the development of vertical stylolites related to the Acadian Orogeny (Middle Devonian). Carbon (C) and oxygen (O) stable isotopes and fluid inclusion microthermometry of the fracture-filling calcite cements indicate significantly higher thermal conditions for the southern domain than the western sector of the central domain. Low thermal maturation and hydrocarbon occurrences in the eastern part of the central domain (e.g. the Mississippi Anticline area) have been documented. These observations are consistent with our new isotopic and fluid inclusion data, which suggests lower thermal conditions compared with adjacent areas. Highly fluorescent hydrocarbon fluid inclusions are present in fracture-filling calcite cement. API values average around 40 °, and GC-MS analysis of a limited volume of decrepitates suggests a Devonian or older marine black shale source rock.

R1~SUMg

Les Calcaires sup6rieurs de Gasp6 du D6vonien Infdrieur comprennent les formations de Forillon, Shiphead et Indian Cove. Le bassin de d6p6t et la succession stratigraphique r6sultante ont 6t6 controlds par des failles de tension syns4dimentaires d4limitant trois domaines pal4og6ographiques majeurs: une plate-forme proximale au nord, une plate-forme distale au centre et un environnement de pied de pente au sud. Des suintements d'huile et une production limit4e d'hydrocarbures sont connus dans les calcaires fractur4s du domaine central.

Une porosit4 primaire fut conserv4e dans des facihs g brachiopodes de la Formation d'Indian Cove du domaine septentrional. Un accroissement du volume de la porosit4 fut reconnu localement et des circulations tardives d'eaux m4tdoriques en seraient responsables, ce qui est sugg6r6 par la nature des ciments de calcite remplissant les fractures. Cet 6v6nement est restreint au domaine septentrional, aucune migration d'hydrocarbure n 'y est reconnue.

De multiples phases de fracturation, de dissolution et de cimentation de calcite du domaine de l'enfouissement sont pr4sents dans les successions des domaines central et mdridional. La p6trographie sugg~re trois dv6nements diag6n~- tiques distincts avant le d6veloppement de stylolites verticaux associds g l'orog6nie acadienne (Ddvonien M6dian). Les donnEes isotopiques du C et de 1'O et les donn6es microthermomEtriques d'inclusions fluides sur les calcites de remplissage de fractures indiquent des conditions thermiques plus 61ev6es pour le domaine mdridional et pour le secteur occidental du domaine central. Le segment oriental de ce dernier (e.g. la rdgion de l'Anticlinal de Mississippi) fut ant6rieurement document6 comme 6rant de maturation peu 61evde et est caract~ris6 par des accumulations

346

THE LOWER DEVONIAN UPPER GASPE LIMESTONES: CARBONATE D1AGENESIS AND RESERVOIR POTENTIAL 3 4 7

d'hydrocarbures. Ces faits concordent avec nos nouvelles donn6es isotopiques et d'inclusions fluides, lesquelles suggbrent des conditions thermiques plus basses que celles des r6gions adjacentes. Des inclusions flnides d'hydrocarbures fortement fluorescentes sont prdsentes dans les ciments de calcite remplissant des fractures. Les valeurs moyennes d 'API sont aux environs de 40 et les analyses GC-MS de volumes limit6s d'huile provenant des inclusions suggbrent, comme roche mbre, un shale marin d6vonien ou plus vieux, fiche en mati~re organique.

Traduit par les auteurs.

I N T R O D U C T I O N

The occurrence of oil seeps in the Lower Devonian Upper Gasp6 Limestones has attracted exploration drilling for more than a century in the northeastern part of the Gasp6 Peninsula, but only a small volume of oil has been produced from these strata. Hydrocarbon seeps are typically located along faults (McGerrigle, 1950). Previous work has documented that, in the areas where oil seeps are known (e.g. Mississippi Anticline area; Fig. 1), the Upper Gasp6 Limestones are charactefised by relatively uniform, fine-grained, distal outer-shelf facies. Primary porosity is close to nil, and hydrocarbons are stored in fractured reservoirs Lavoie, 1996a; Lavoie and Nadeau, 1996).

/ 66 ~

St. Lawrence River

Organic matter diagenesis and clay mineralogy of the Upper Gasp6 Limestones were systematically studied by Bertrand (1987, 1996). These thermal indicators suggest that, for the most part, the unit is within or close to the oil window in the eastern part of the Gasp6 Peninsula (Bertrand and Malo, 2001, this issue). In contrast, carbonate diagenesis is largely unknown. The only previously published studies on carbonate diagenesis are those of Lavoie and Bergeron (1993) and Lavoie (1993, 1994). The latter study describes the reconstructed stable isotope composition of the Early Devonian Gasp6 Basin sea. The Upper Gasp6 Limestones are predominantly finely crystalline, and little effort has been made to understand their

/ 65 °

Sample Locations

Northern domain Central domain 1. Forillon Peninsula 3. MissisippiAnticfine 2. Road 132 4. Gait No.1 well

5. Sunny Bank well

Southern domain 6. Oatcake Creek J 7. Riviere St-Jean Anticline I 8. Bazire Creek 9. Grande R v ere

F(~rtin/T~rn'iscouata groups E~] Chaleurs Group

Matapedia and Honorat/Cabano groups

~Z] Quebec Supergroup

348 D. LAVOIE, G. CH1 and M. FOWLER

diagenetic evolution. The tectonodiagenetic evolution of abundant calcite-filled and partially open fractures has been the focus of industry workers (Lavoie, 1996a, b; Lavoie and Nadeau, 1996; Lavoie and Chi, 1997).

This paper describes the evolution of primary and secondary

(fracture and dissolution) porosities in the Upper Gasp6 Limestones. Interpretations are based on petrographic, fluid-

inclusion, and O and C stable isotope data gathered from various cement fillings. Hydrocarbon charge is also described, based on the character of oil fluid inclusions in some cement

phases. Because pore-filling cements are scarce, fracture-filling cements are focussed on in this study.

G E O L O G I C A L S E T T I N G

The study area, in the lower Paleozoic Gasp6 Belt (sensu Bourque et al., 1995), is located in the northeastern part of the

Qurbec segment of the Appalachian Orogen (Bourque et al., 2001, Figs. 1, 2, this issue). The Gasp6 Mobile Belt comprises sedimentary rocks formed in numerous tectonic settings

(Bourque et al., 1995). Carbonate sedimentation occurred during tectonic activity and quiescence, forming carbonate shelves

that range in age from latest Ordovician to Early Devonian

(Bourque et al., 1986; Lavoie, 1992a, b). During the Early Devonian, this part of the Appalachian Orogen was located at about 18 ° south (Van der Voo, 1988).

The Upper Gasp6 Limestones outcrop in the eastern part of the Silurian-Devonian Connecticut Valley-Gasp6 Synclinorium of the Gasp6 Belt (Bourque et al., 1995) (Fig. 1). The Connecticut Valley-Gasp6 Synclinorium lies between the Cambro-Ordovician Taconian allochthons to the north and the Middle Ordovician-earliest Silurian Aroostook-Perc6 Anticlinorium to the south (Bourque et al., 2001, this issue). The Connecticut Valley-Gasp6 Synclinorium is an Acadian (Middle Devonian) structure (Malo and Brland, 1989) dissect- ed by a number of faults. In eastern Gasp6 Peninsula (Fig. 1), the Bassin Nord-Ouest and Troisi~me Lac faults are dextral strike-slip faults featuring 7 to 8 km stratigraphic offsets (Brisebois, 1981; Kirkwood, 1986). These structures are reacti- vated listric normal faults that were active in Early Devonian time (Roksandic and Granger, 1981; Bourque, 1990; Lavoie, 1992a; Malo and Bourque, 1993; Bourque, 2001, this issue; Malo, 2001, this issue).

The Upper Gasp6 Limestones conformably overlie the Lower Silurian to Lower Devonian (Lochkovian) Chaleurs Group (Bourque et al., 2001, this issue). The Upper Gasp6 Limestones are conformably overlain by the Lower Devonian

-- 381 i

Eifelian

- - 386- -

Emsian

- - 3 9 0 - -

Pragian

a -- 396--

Lochkovian

409 --

Pridolian _=1

Sea-level curve ( FALL RISE ) Upper Gasp@ • ~ Limestones

t {Forillon ~ ...... Peninsula

,' ~ Gasp~ ~ ,, ,

Sandstones ~ ,, 8r:o - / 8 i.,~, I I IIII l l l l l I I l l l l

I I I I I (~ UpperGasp6 ~ ~ I":l"=i":;J': . . . . . . . . . . I I I - n ~ ' l~ _ ......................................... Limestones ./

Chaleurs I i i

Group / :" 1 I00 m

10

BRACHIOPODS CHITINOZOANS {Lesp~rance, 1980] {Achab et al., 1997

I Elymothyris Zone

Rennseloeria Zone

E m s i a n

? E m s i a n

?erag ian

l o w e r e r a g i a n

l o w . e r a g i a n L up, pe r .

o c n K o v l a n

u p p e r L o c h k o v i a n

Modified from ~ ] c o n g l o m e r a t e ~ m u d s t o n e - ~ i c a l c a l e ° u s ~ chert ~ dolomitlc .. sandy

Bourque etal . {1995] IS ]sandstone E~]carbonate ~ nodular - shaly

b=benthonic clay

Fig. 2. Upper Silurian-lower Middle Devonian stratigraphy of the eastern segment of the Gaspe Belt. The Upper Gaspe Limestones were deposited during the highstand of a second-order transgressive-regressive cycle. The detailed stratigraphic column of the Upper Gaspe Limestones is based on Lavoie (1992b) and refers to the northern domain succession in Forillon Peninsula. CG Mb = Cap Gaspe Member, MSA Mb = Mont St AIban Member.

THE LOWER DEVONIAN UPPER GASPt~" LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 349

(Emsian)-Middle Devonian (Eifelian) Gasp6 Sandstones (Bourque et al., 2001, this issue). In central Gaspr, the Upper Gasp6 Limestones are laterally equivalent to the siliciclastics of the Fortin Group, likely deposited below storm wave base (Lavoie, 1992a). The Upper Gasp6 Limestones constitute one of the rare carbonate units in the otherwise siliciclastic-dominated, post-Taconian succession (Bourque et al., 1995). These carbonates accumulated during a relative sea-level highstand within a 20 Ma transgressive-regressive cycle (Lavoie, 1993) (Fig. 2) comprising three tectonically controlled, third-order cycles (Fig. 3). Higher-order cyclic sedimentation has previously been documented empirically (Lavoie, 1992b) and statis- tically (Achab et al., 1997; Mussard and Lavoie, 1997).

The Forillon Peninsula of eastern Gasp6 Peninsula (Fig. 1) is the type area of the Upper Gasp6 Limestones. There, three units are recognized by Lesprrance (1980): the Fofillon, Shiphead and Indian Cove formations. These lithostratigraphic units are

recognized throughout eastern Gaspr. However, south of the Bassin Nord-Ouest Fault (Figs. 1, 4), the succession is thicker and is chiefly a monotonous succession of lime mudstone and wackestone. South of the Troisirme Lac Fault (Fig. 1), the Upper Gasp6 Limestones are thicker and comprise impure limestone with abundant rotational and translational slide structures that interfinger with the coarse- to fine-grained siliciclastics of the Fortin Group (Rouillard, 1986; Lavoie, 1990, 1992b; Lavoie et al., 1990, 1991). Based on sedimentological analyses, three paleogeographical domains and associated facies are recognized, and their distribution was likely controlled by the Bassin Nord-Ouest and Troisi~me Lac faults. From northeast to southwest, they are (1) the northeastern proximal outer shelf domain; (2) the central, gently sloping, distal outer shelf domain; and (3) the southwestern slope and toe-of-slope domain (Fig. 4). Each of these domains contains a distinct facies belt that is described according to the paleogeographic

GASPI~ SANDSTONES

~°

o - - _ _ _ ~ CHALEURS GROUP [Indian Point Fm]

STORM DEPOSITS PACKSTONE/GRAINSTONE BEDS

coarse.grained % Storm-beds

0 10 20 30%

(a)

A X I S 2 OF C,J~. Fall Rise ~, S M O O T H E D C O O R D I N A T E S

T ~"""" '='=l' ?all Ri,~T

1i imP)

Fig. 3. Cyclic third-order sedimentation recorded in the Upper Gasp6 Limestones. This cyclicity is empirically recognized on the basis of (A) lithofacies analyses (Lavoie, 1992b) and (B) statistical treatment (correspondance analysis = C.A.) of lithofacies data (Achab et al. 1997; Mussard and Lavoie 1997).

350 D. LAVOIE, G. CHI and M. FOWLER

setting. For example, domain (1) contains a proximal outer- shelf facies. Active faulting may have influenced the distribution of Upper Silurian to Lower Devonian sedimentation in the eastern part of the Gasp6 Belt (Amyot, 1984; Lavoie, 1992a; Malo and Bourque, 1993; Achab et al., 1997; Mussard and Lavoie, 1997; Bourque, 2001, this issue). The proximal outer shelf domain contains the shallowest water facies of Pragian age on the Gasp6 Peninsula (Lavoie, 1992b). There is a significant southward increase in thickness of the unit, from an average of 500 m in the northern, proximal outer shelf domain to nearly 2 km in the southernmost toe-of-slope domain (Lavoie, 1992a).

The proximal outer shelf facies consists predominantly of siliceous to cherty lime rnudstone with subordinate bioclastic wackestone and bioclastic and intraclastic packstone-grain-

•... @rlllon

,2.o

Perce

[~ Proximal outer shelf 0 so km

Distal outer shelf

• • Slope / t o e of s lope

G R T L B N O ^ A'

I

• 0

I

2k in

20 km

Upper Gasp@ Limestones ~ Fortln Group

Chaleurs Group

Fig. 4. Paleoenvironmental interpretation of lithotectonic domains of the Upper Gaspe Limestones in eastern Gaspe. The three depositional belts are limited by Acadian (Middle Devonian) strike-slip faults. The schematic cross-section (A-A') shows the relationships between synsedimentary faulting and lithofacies. Modified from Achab et al. (1997). BNO = Bassin Nord-Ouest Fault, TL = Troisieme Lac Fault, GR = Grande Rivi~re Fault.

stone (storm beds) and sandy limestone (Lavoie 1992b; Mussard and Lavoie 1997). The distal outer shelf facies consists predominantly of impure to locally siliceous lime mudstone, with minor, thin bioclastic wackestone and packstone and some volcanics and volcaniclastics (Lavoie et al., 1991; Lavoie, 1992a, 1995). The toe-of-slope facies belt consists of impure lime mudstone interbedded with debris-flow units and coarse-grained siliciclastic beds (Lavoie et al., 1990; Lavoie, 1992a).

STRATIGRAPHY

FORILLON FORMATION

In the area north of the Bassin Nord-Ouest Fault, the Forillon Formation (Fig. 2) comprises two members: the Mont Saint- Alban and the Cap Gasp6 members (Lavoie et al., 1990). The Mont Saint-Alban Member contains siliciclastic mudstone beds 3 to 50 cm thick with uncommon interbedded shaly lime mudstone and wackestone. The mudstones are poorly fossiliferous, although a few trilobites and brachiopods of Lochkovian age occur (Lesp6rance, 1980; Bourque et al., 1995). The Cap Gasp6 Member consists of wavy bedded, lime mudstone and wackestone beds 5 to 50 cm thick. These carbonates are slightly or pervasively silicified. The wackestone contains well-preserved macrofauna, including abundant trilobites and brachiopods of the Rennselaeria (Lower Devonian) Zone (Lesp6rance and Sheehan, 1975; Lesp6rance, 1980). Scattered packstone beds less than 50 cm thick also occur. South of the Bassin Nord- Ouest Fault, the formation consists of a thick succession of argillaceous lime mudstone beds 5 to 15 cm thick, with thin shale partings.

SHIPHEAD FORMATION

North of the Bassin Nord-Ouest fault, the Shiphead Formation (Fig. 2) is a heterogeneous unit of interbedded impure carbonate (85%), fine- to coarse-grained siliciclastics (10%), and bentonitic clay (5%). Silty lime mudstone and wackestone occur in beds up to 50 cm thick. The macrofauna is similar to that of the Cap Gasp6 Member. Sandy packstone and grainstone interfinger with the lime mudstone and wackestone in the upper few metres of the unit. South of the Bassin Nord- Ouest Fault, the formation is similar to the Forillon Formation, although with significantly more argillaceous and silty carbonates.

INDIAN COVE FORMATION

In the northern facies belt, the Indian Cove Formation (Fig. 2) consists of siliceous to cherty lime mudstone and wackestone in wavy beds 10 to 35 cm thick. About 20% of the formation contains interbeds of thin-bedded argillaceous lime mudstone that are commonly silicified. The wackestone contains abundant fauna, mostly trilobites and brachiopods, including species of the Etymothyris (Emsian) Zone in the uppermost part of the formation (Lesp6rance, 1980). A significant volume (about 20% of the unit) of cement- and allochem-rich packstone and grainstone beds (15 to 45 cm thick) occurs in the upper half

THE LOWER DEVONIAN UPPER GASPE LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 351

of the formation. South of the Bassin Nord-Ouest Fault, the formation is a well-bedded, locally siliceous lime mudstone and wackestone (5 to 20 cm thick) with thin shale partings.

RESEARCH METHODS

In order to study the diagenetic evolution of the Upper GaspE Limestones, specific outcrop sections and drill cores were described and sampled (Fig. 1). The representative sections were chosen from the three domains: the northern (Forillon Peninsula and Road 132), central (Mississippi Anticline, Gait No.1 Well and Sunny Bank Well), and southem (Oatcake Creek, Rivirre St-Jean Anticline, Bazire Creek, and Grande Rivibre) domains (Fig. 1). These sections are considered representative of the diverse lithotypes and regional diagenesis of the Upper GaspE Limestones. Details of the sections are described in Lavoie et al. (1990, 1991) and Lavoie (1992a, b, c).

A total of 74 samples was collected (26 in the Forillon, 6 in the Shiphead Formation and 42 in the Indian Cove formations). Few samples were collected from the Shiphead Formation, because the unit is poorly exposed. From these samples, 61 standard thin sections were made (22 for the Forillon, 5 for the Shiphead and 34 for the Indian Cove). In addition, 30 polished thin sections for cathodoluminescence (CL) petrography were prepared (14 for the Forillon, 2 for the Shiphead and 14 for the Indian Cove). Oxygen and carbon stable isotope analyses were done on 57 samples (13 for the Forillon, 3 for the Shiphead and 41 for the Indian Cove). Finally, fluid inclusion studies were done on 11 double-polished thin sections (5 for the Forillon, 1 for the Shiphead and 5 for the Indian Cove). Hydrocarbon fluid inclusions from two samples of the Indian Cove Formation were analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS).

Pore- and fracture-filling cements were studied under light and cathodoluminescence (CL) microscopy. Generations of cements were differentiated based on crosscutting relationships and relationships with other diagenetic features, such as stylolites. Samples for C and O isotope analyses were micro-drilled from parent rock slabs, and integrity of individual cement phases was further checked under CL for the milled areas. The carbonate powders were treated at the Delta-Lab [Geological Survey of Canada (GSC) (QuEbec)] following standard procedures. Results are expressed in the usual delta notation and given in per nail relative to the VPDB standard. Precision of the data is better than _+0.1%o for both 6180 and 613C.

Microthermometric measurements on fluid inclusions were carried out with a USGS heating/freezing stage made by Fluid Inc. The homogenization temperature (Th) and final ice-melting temperatures (Tm-ice) of aqueous fluid inclusions were measured with a precision of _+I°C and _+0.2°C, respectively. Oil inclusions were routinely described for fluorescence using a Zeiss II photomicroscope. Some fluorescence spectra (400-700 nm) of oil inclusions were measured at the GSC (Calgary). Details of the method are described in Stasiuk and Snowdon (1997). Two calcite veins containing abundant oil inclusions were hand-

picked for GC and GC-MS analyses. The veins were suffi- ciently wide to allow elimination (by hand picking) of host rocks in the sample. Aliquots of dried powdered rock (about 100 mesh) were extracted using azeotropic chloro- form:methanol (87:13) for 24 hours. The extracts were fractionated using open column chromatography (3/4 activated alu- mina and 1/4 activated silica gel with an adsorbent:sample mass ratio of 100:1). GC and GC-MS analyses were made at the GSC (Calgary) following the procedures described in Fowler et al. (1995).

PETROGRAPHY

CALCITE-FILLED BRACHIOPOD-RICH BEDS

Beds of packstone and locally grainstone, very rich in brachiopods, occur only in the northern proximal outer-shelf domain (Road 132 and Forillon Peninsula, Fig. 1). They are restricted to the uppermost part of the Indian Cove Formation, in the top of metre-scale, shallowing-upward cycles (Lavoie, 1992b; Achab et al., 1997). These beds consist of densely packed shells in a muddy to sparry matrix (Fig. 5A) and show visible porosity as intra-brachiopod cavities, partly filled by various cements (Lavoie, 1992b, 1996b).

The intergranular and intraparticular cements consist of non- ferroan calcite with, in the case of large pores, some later ferroan calcite. A euhedral silica cement rarely caps this cement succession. The calcite cements occur as 0.1 to 1 mm blocky crystals that coarsen toward the centre of the pores (Fig. 5B). CL petrography reveals that the non-ferroan calcite cement consists of an initial, relatively thin, virtually isopachous rind of non- to blotchy-luminescent calcite coating the internal wall of the brachiopod shells (Fig. 5C). This primary pore-filling, non- luminescent calcite (PF-NL) cement contains microdolomite inclusions and is likely a diagenetic low-magnesium calcite after a precursor high-magnesium calcite precipitate (Lohmann and Meyers, 1977), The PF-NL cement is conformably overlain by zoned dull-bright luminescent calcite (PF-zoned) (Fig. 5C). The later ferroan calcite is darker dull luminescent (PF- dull). The PF-zoned and PF-dull cements are rhombus-shaped. Well-preserved crystal tips pointing toward the centre of open voids can be seen locally, which indicate preservation of primary pore space. In places, corrosion of crystal tips is observed, which suggests some enlargement of pore space by dissolution (Fig. 5C).

FRACTURE-FILLING CALCITE CEMENTS

Fracture-filling calcite samples were collected from the three domains. Three distinct fracture systems and calcite fillings are recognized in the samples from the distal outer shelf (central domain) and the slope and toe-of-slope (southern domain). Another distinct system has been documented in the samples from the proximal outer shelf (northern domain), although its time relationships with the other fracture systems is unclear.


Fig. 5. A. Brachiopod-rich packstone with some intrabrachiopod open pore space. Indian Cove Formation, Road 132 section, northern domain. B. Photomicrograph in plane-polarized light of intrabrachiopod pore-filling calcite cements showing an initial rind of inclusion-rich lamel- lar radiaxial calcite (LRC) followed inward by coarse-crystalline inclusion-rich to inclusion-poor calcite spar. C. Photomicrograph under CL of intrabrachiopod pore-filling calcite cement showing the initial rind of non- to blotchy luminescent calcite (BLC) overlain by zoned, dull-bright luminescent calcite (PF-zoned). Corrosion of crystal tips is locally observed (arrows). Scale bars = 0.1 mm.

Northern Domain

The best developed fracture system occurs in the Indian Cove Formation, near the Forillon Peninsula. Samples were collected in order to document the relationship of fracturing and dissolution-enhanced primary porosity in the brachiopod-rich beds. Fractures are millimetre- to centimetre-scale and locally display irregular margins. From the limited number of samples, fractures appear to post-date stylolites. Fracture-filling calcite consists of large, blocky, cloudy crystals with small inclusion- poor crystals as final precipitates. No open pore space occurs in the sample studied. Under CL, the calcite cement consists of strongly zoned bright and dull luminescent idiomorphic crystals (Fig. 6). This calcite cement is called fracture-filling zoned calcite (FF-zoned).

Central and Southern Domains

Samples of these domains contain a complex network of fracture-fill cements associated with dissolution. A first fracture system consists of millimetre- to centimetre-sized fractures that developed mainly after bedding-parallel stylolites, but locally at the same time as the stylolites. The fractures are filled with large xenomorphic to idiomorphic cloudy calcite cements. Under CL, the cement is orange in luminescence (FF-orange) and is locally faintly zoned (Fig. 7A). In many places, dissolution followed this cement phase (Fig. 7B) with the resulting voids filled by later calcite cement (see below). The margin of the dissolution voids is locally the site of bitumen concentration.

The second fracture set consists of millimetre- to centimetre- sized fractures that clearly post-date stylolite formation. Small idiomorphic, limpid (inclusion-poor) calcite cements fill fractures. Under CL, this cement, which is also present in dissolution voids cutting the calcite cement of the previous fracture set, consists of dull to very dark luminescent calcite (FF-dull), with some non-luminescent growth bands (Figs. 7, 8A). Again, in places, dissolution post-dates this calcite, with the resulting voids filled with a later calcite cement.

The third and last fracture set is represented by rare submil- limetre-sized fractures that post-date horizontal, but pre-date vertical, stylolites. Small, idiomorphic, inclusion-poor calcite crystals fill the fractures. Under CL, this cement, which also fills some dissolution voids in previous fractures, consists of bright luminescent calcite spar (FF-bright) (Fig. 8B). No visible void space is associated with this fracture set. Therefore, dissolution porosity was generated in the fracture-fill calcite cements. This porosity was either filled by various calcite cements or remained open; however, open space is uncommon and consists of millimetre- to rarely centimetre-sized cavities. The degree of cavity connection is unknown, but local hydrocarbon storage and production suggest that a relatively efficient permeable framework is present, at least locally.

STYLOLITES

Stylolites are common in the Upper Gasp6 Limestones. More than 95% of all stylolites are either roughly parallel or slightly inclined to bedding planes, with the remaining oriented

THE LOWER DEVONIAN UPPER GASPr~ LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 353

at right angles to bedding planes. Most of the stylolites consist those from the southern domain (Grande Rivibre and St. Jean of small amplitude nonsutured seams. High amplitude sutured stylolites are particularly rare.

Bedding-parallel stylolites pre-date the fractures, although some are coeval with the first phase of the burial fracture system. Bedding-perpendicular stylolites clearly post-date fractures and have been shown to act as migration conduits for some hydrocarbons (Bertrand, 1987, 1996).

River anticlines) range from -8.4 to -13.9%0. A similar trend is observed for the Indian Cove Formation (see below).

SttIPHEAD FORMATION

The results for the Shiphead Formation are shown in Table 1 and Figure 10. Only three analyses were made, all from the Mississippi Anticline samples. Two analyses of the

STABLE ISOTOPES

Oxygen and carbon isotope analyses of the various cement phases are listed in Table 1. The range of d180 values is broad (-2.6 to -13.9%o), whereas that of d13C is relatively narrow (-1.5 to +3.2%o). Results for each of the three formations are discussed separately (Figs. 9-11).

FORILLON FORMATION

The results for the Forillon Formation are shown in Table 1 and Figure 9. Four analyses of the fracture-filling, orange luminescent calcite (FF-orange) yielded d180 values ranging from -5.1 to-8.7%~ (average: -6.9%o) and d13C values from +1.7 to +3.0%o (average: +2.5%0). Nine analyses of the fracture-filling, dull luminescent calcite (FF-dull) yielded dlSO values ranging f rom-5.5 to -13.9%o (average: -9.1%o) and dl3C values from +0.8 to +2.9%o (average: +2.3%0).

When results are grouped by domains, an obvious southerly decrease in d180 values is noted (Fig. 9). The d180 values of FF-dull calcite range from -5.5 to -8.0%0 in the central domain (Mississippi Anticline, Galt and Sunny Bank wells), whereas

Fig. 6. Photomicrograph under CL of a fracture cutting through a lime mudstone of the Indian Cove Formation at Forillon Peninsula. The fracture is filled by strongly zoned bright and orange, dull luminescent calcite cement. Scale bar = 0.1 ram.

Fig. 7. A. Photomicrograph under CL of large idiomorphic calcite cement coating the wall of a fracture (arrows). The cement is orange, dull luminescent (FF-orange), faintly zoned, and overlain by large xenomorphic dull luminescent calcite cement. B. Photomicrograph under CL of orange, dull luminescent calcite crystals that have been intensely cor- roded and dissolved (arrows), with the resulting pore space being filled by a dark, dull luminescent calcite phase. Scale bars = 0.1 mm.

354 D. LAVOIE, G. CHl and M. FOWLER

fracture-filling, orange luminescent calcite (FF-orange) yielded 8180 values of-6.6%o and -8.2%o with respective 813C values of +2.7%o and +2.6%0. A fracture-filling, dull luminescent calcite (FF-dull) provided a 8180 value of-7.3%o with a 813C value of +2.7%o. Although limited, the results from the Shiphead Formation agree with the range of relatively high 8180 values that characterize the central domain.

Fig. 8. A. Photomicrograph under CL showing a fracture (dashed- outline) filled by dull luminescent calcite with non-luminescent (NL) growth bands. B. Photomicrograph under CL showing a late fracture filled by bright luminescent calcite spar (FF-bright). The fracture cuts through an early orange, dull luminescent, fracture-filling calcite cement (FF-orange). Scale bars = 0.1 mm.

INDIAN COVE FORMATION

Samples for C and O isotope analyses for the Indian Cove Formation come from all three domains. In the northern domain, analyses were made on pore-filling calcite (PF-NL, PF-zoned, PF-dull) and fracture-filling calcite (FF-zoned). In the central and southern domains, fracture-filling calcites were analyzed (FF-orange and FF-dull). The results are listed in Table 1 and shown in Figure 11.

Nor thern D o m a i n

Three analyses of the non- and blotchy luminescent calcite cement (PF-NL) coating the walls of brachiopod shells have yielded 8180 values of -2.6, -2.9 and -4.2%0, with respective 813C values of +2.8, +2.0 and +1.5%o. These values are similar to those given by Lavoie (1993) from non-luminescent brachiopod shells which had 8180 values ranging from-2.7 to --4.6%o and 813C values from +1.4 to +1.7%o. Two analyses of the suc- ceeding, strongly zoned luminescent calcite (PF-zoned) yielded ~180 values o f -9 .2 and -9.9%0, with respective 813C values of +1.3 and +1.2%o. Finally, three analyses of the following dull luminescent calcite cement (PF-dull) have 8180 values of-10.6, -10.8 and -11.1%o with 813C values of +0.8, +0.8 and +0.7%o, respectively.

Four analyses of the fracture-filling, strongly zoned, bright, and dull luminescent calcite cement (FF-zoned) yielded 8180 values ranging from -11.2 to -12.5%o (average: -11.9%0 with 813C values from -1.5 to +0.3%o (average: -0.5%0). The deple- tion in 13C relative to the marine calcite field (Lavoie, 1993) suggests a source other than marine for bicarbonates. The low 8180 values suggest either precipitation at relatively high temperature or from fluids depleted in ~80.

Central and S o u t h e r n D o m a i n s

Fourteen analyses of the orange luminescent calcite cement (FF-orange) from the central and southern domains yielded 8180 values from -4.7 to -11.3%o (average: -7.5%0 and 813C

1_34C VPDB

k " . . . . . . [] '

J '~ Southem ./~ ~¢L / - ' , , , domain I \ '~.~ \ 2 / " . \

1-18 I I I I I I L 2 ~ 0 -14 -12 -10 -8 -6 -4 -2

~ FF-orange VPDB

FF.du, -1

Fig. 9. Crossplot of 8180 and 813C values for the Forillon Formation. Fracture-filling orange luminescent calcite (FF-orange) samples are all from the central domain (Mississippi Anticline area). Results for fracture- filling dull calcite (FF-dull), grouped according to the sampling areas, show an obvious southerly decrease in 8180. The shaded box is the Early Devonian marine calcite field from Lavoie (1993).

THE LOWER DEVONIAN UPPER GASPI~ LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 355

from +0.6 to +3.2%o (average: +1.7%o). Twelve analyses of the dull luminescent calcite cement yielded 6isO values ranging f rom-5 .1 to -12.5%o (average: -8.7%0) and 513C values from +1.2 to +3.2%o (average: +2.1%o). The broad range in 6180 values is partly related to locality. As in the Fofillon Formation, a southerly decrease in 8180 values is observed, although there is a large overlap between the central and the southern domains (Fig. 11).

FLUID INCLUSIONS

Fluid inclusions were studied in the fracture-filling, strongly zoned luminescent calcite (FF-zoned) from the northern domain, and in the orange (FF-orange) and dull (FF-dull) luminescent calcites from the central and southern domains. Workable aqueous inclusions occur in most samples. Oil inclusions were found in two samples from the Indian Cove Formation (one from the Mississippi Anticline area and central domain, and the other from the Oatcake Creek area or southern domain). The microthermometric results of both aqueous and oil inclusions from the Forillon and Indian Cove cements and the geochemistry of oil inclusions are discussed in this section.

MICROTHERMOMETRY OF AQUEOUS INCLUSIONS

Fluid inclusions that are either isolated, randomly distrib- uted, or occurring in clusters, are considered as primary or pseudo-secondary. The results of microthermometric measurements of aqueous inclusions are listed in Table 2. The overall range of homogenization temperatures (Th) is from probably <50°C (for all-liquid inclusions) to 169.2°C (Table 2 and Fig. 12). Most of the Th results are lower than 100°C. These temperatures are relatively low in comparison to values from older stratigraphic units (e.g. Th up to 226°C in the Sayabec Formation; Lavoie and Chi, 2001, this issue). All-liquid inclusions (i.e. no vapour bubbles at room temperature) occur in many of the examined samples. In most cases, these inclusions do not coexist with liquid + vapour inclusions, suggesting that they do not result from necking of the latter. In other cases, where all-liquid inclusions coexist with liquid + vapour inclusions, there is no petrographic evidence for necking and the Th of the coexisting liquid + vapour inclusions is usually around 50°C (See Table 2). The all-liquid inclusions were probably entrapped at temperatures lower than 50°C. The homogenization temperatures of aqueous fluid inclusions are higher in the late calcite cements and tend to increase from the central to southern domains (Fig. 12).

The range of salinity values is from 4.2 to 23.9 wt % NaC1- equivalent, i.e. higher than sea water salinity (-3.5 wt %), except for one FF-zoned sample (UGL-3, Table 2) from the northern domain, where low salinity values (0.3 - 2.1) were recorded (Table 2 and Fig. 13). It is suggested that all the FF- orange and FF-dull calcites were precipitated from basinal fluids. The low salinity found in FF-zoned calcite from the northern domain possibly reflects a contribution from meteoric water during the precipitation of the calcite, because the FF-zoned calci te is also characterized by relatively low 5180 and ~13C

values (Table 1 and Fig. 11). It is inferred that the FF-zoned calcite was precipitated in a mixing zone between meteoric water and basinal fluids.

CHARACTERISTICS OF OIL INCLUSIONS AND GEOBAROMETRY

A number of oil inclusions occur in FF-dull calcite in a sample from the Mississippi Anticline area in the central domain (UGL-33, Table 2) and in a sample from the Oatcake Creek area in the southern domain (UGL-57, Table 2). These oil inclusions are colourless under transmitted light (Fig. 14). Under blue light excitation, they strongly fluoresce. The fluorescence colour ranges from blue-tinted white to white and yel- lowish white. Fluorescence spectra of oil inclusions from UGL- 33 show Lmax values from 430 to 500 nm and Q (=

~I~4VPDB

,ll, O O

I! ¸ I

I I I I I -10 -8 -6 -4 -2

g FF-orange .... FF-dul l

-1

2 VPDB

Fig. 10. Crossplot of 5180 and 5130 values for the Shiphead Formation. The entire data set is from the Mississippi Anticline. The shaded box is the Early Devonian marine calcite field from Lavoie (1993).

• ~oum \ I domain / / ~1~ O Central \ I / ~ domain t I / i ~ |

I <-'

• o

; "1 OI I I I iO.12 OJ -70 -8 -6 -4

I North # idomaln/ I x PF-NL O FF-zoned

z PF-zoned O FF-orange ~x O / / + PF-dull • FF-dull

13 6 %VPDB

3 x

2

1

-2 VPDB

-1

-2

Fig. 11. Crossplot of 5180 and 5130 values for the Indian Cove Formation. The shaded box is the Early Devonian marine calcite field from Lavoie (1993).


Table 1. Results of carbon and oxygen isotope analyses of calcite cements.

Unit Sample No. Locali ty* Cement phase** 81~ 8t~ Forillon Fm UGL-16 MA FF-orange 2.5 -5.1

UGL-18 MA FF-orange 1.7 -8.7 UGL-19 MA FF-orange 3.0 -6.4

GALT4-1 GALT FF-orange 2.6 -7.2 FF-orange (average) 2.5 -6.9

UGL-18 MA FF-dull 0.8 -5.5 GALT4-2 GALT FF-dull 2.7 -7.5 GALT4-3 GALT FF-dull 2.6 -7.5

SB8 SB FF-dull 2.9 -8.1 SB8 SB FF-dull 2.9 -8.0

UGL-38 GR FF-dull 1.0 -9.3 UGL-39 GR FF-dull 2.4 -13.8 UGL-40 GR FF-dull 2.5 -13.9 UGL-48 RSJ FF-dull 2.9 -8.4

FF-dull (average) 2.3 -9.1 Shiphead Fm UGL-26 MA FF-orange 2.6 -8.2

UGL-26 MA FF-orange 2.7 -6.6 UGL-24 MA FF-dull 2.7 -7.3

Indian Cove Fm 2-33 FP PF-NL 2.0 -2.9 2-34 FP PF-NL 2.8 -2.6 2-33 FP PF-NL 1.5 -4.2

PF-NL (average} 2.1 -3.2 2-60 FP PF-zoned 1.2 -9.9 IG-3 FP PF-zoned 1.3 -9.2

PF-zoned (average) 1.3 -9.5 2-60 FP PF-dull 0.8 -10.6 2-97 FP PF-dull 0.7 -11.1 2-60 FP PF-dull 0.8 -10.8

PF-dull (average) 0.8 -10.8 UGL-3 ROAD 132 FF-zoned -1.5 -12.4 UGL-3 ROAD 132 FF-zoned -0.5 -12.5 IF16 FP FF-zoned -0.2 -11.3 1F16 FP FF-zoned 0.3 -11.2

FF-zoned (average) -0.5 -11.9 SB10 SB FF-orange 3.2 -7.6 SB2 SB FF-orange 1.6 -7.0 SB9 SB FF-orange 2.9 -7.6 SB6 SB FF-orangE 2.1 -8.8

GALT1-2 GALT FF-orange 0.6 -6.3 GALT1-2 GALT FF-orange 1.5 -6.2 GALT1-3 GALT FF-orange 0.8 -6.6 UGL-35 BC FF-orange 1.7 -8.6 UGL-36 BC FF-orange 1.5 -11.3 UGL-44 GR FF-orange 3.1 -9.0 UGL-46 GR FF-orange 1.2 -4.9 UGL-46 GR FF-orange 1.2 -4.7 UGL-56 OC FF-orange 1.1 -7.8 UGL-56 OC FF-orange 1.0 -8.4

FF-orange (average) 1.7 -7.5 SB5 SB FF-dull 2.5 -7.6

SB10 SB FF-dull 3.2 -7.5 SB2 SB FF-dult 2.0 -6.4 SB9 SB FF-dull 3.1 -6.7 SB9 SB FF-dull 2.9 -8.2 SB6 SB FF-dull 2.1 -8.7

GALT1-2 GALT FF-dull 1.2 -10.1 UGL-33 MA FF-dull 1.3 -7.2 UGL-29 MA FF-dull 1.4 -5.1 UGL-44 GR FF-dull 3.0 -12.5 UGL-55 OC FF-dull 1.4 -12.2 UGL-55 OC FF-dull 1.2 -12.3

FF-dull (average) 2.1 -8.7

* MA = Mississippi Anticline, GALT = Gait No. 1 well, SB = Sunny Bank No. 1 well, GR = Grande Riviere, RSJ = Riviere St-Jean FP = Forillon Peninsula, BC = Bazire Creek, OC = Oatcake Creek, ~* FF = fracture-filling, PF = primary pore-filling (see text for details)

THE LOWER DEVONIAN UPPER GASPE LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 357

Intensity650,m/Intensitys00 nm) values from 0.106 to 0.426. API values calculated from Lmax and Q (Stasiuk and Snowdon, 1997) range from 35.0 to 46.3, with an average of 39.7. An average API value of 41.1 is estimated for oil inclusions in sample UGL-57. There is no petrographic evidence for multiple generations of oil inclusions, and the variation in fluorescence characteristics within individual samples is likely to have resulted from different degrees of interaction between oil and aqueous fluids.

The homogenization temperatures of the oil inclusions are fairly convergent within individual samples (Table 1 and Fig. 15). The homogenization temperatures of coexisting aqueous inclusions are slightly (UGL-33) to significantly (UGL-57) higher than those of oil inclusions (Fig. 15). The average Th value of oil inclusions is 50.3°C for UGL-33 and 62.9°C for UGL-57. The average Th value of aqueous inclusions is 65. I°C (not including all-liquid inclusions) for UGL-33 and 162.4°C for UGL-57. The isochores of the oil inclusions could be calculated from the homogenization temperature, the vapour-toliquid

(A) Forillon Formation

' 4 t c--,--n 2 N o FF-orange 0 , , g . , . . . . , , ,

0 50 1 O0 150 2 0 0 m

IJ. Southern domain 4 t El FF-o range FF-dul l

i7 Hn,H 0 . . . . 1 = 6 l o o 15o 200

Homogenization Temperature (°C)

( B ) I n d i a n C o v e F o r m a t i o n

8 O3

E

8 J IJ-

Z

4 ~ Northern domain t ~ FF-zoned 2 al l4 iquid FIs

0 ..... ~ ,~J ~P,I . . . . I I I 0 50 1 O0 150 2 0 0

4 t H Central domain 2 D FF-dull 0 g , FI rq . . . . . . I I t

0 50 1 O0 150 200

Southern domain

! t ~ H c3 FF-orange E3 FF-dull

........ ~ , I - I . . .M, , ,M . . . .

0 50 1 O0 150 2 0 0


ratio and the composition of the oil (Aplin et al., 1999). Without knowing the vapour-to-liquid ratio and the composition of the oil, we have used the average API and Th values to construct the isochores of the oil inclusions with the VTFLINC program of Calsep A/S (Fig. 16) (Chi et al., 2000). With the trapping temperatures being approximated by the average Th values of aqueous inclusions (e.g. Aplin et al., 1999), fluid pressures are calculated to be 341.5 bars for UGL-33 and 763.1 bars for UGL- 57 (Fig. 17).

GEOCHEMISTRY OF O I L INCLUSIONS

An attempt was made to extract the hydrocarbons observed in the oil inclusions in samples UGL-33 and UGL-57. The resulting extracts were fractionated and the saturate fractions analyzed by gas chromatography (GC; Fig. 16) and gas chromatography-mass spectrometry (GCMS; Fig. 18).

Very small amounts of extract were recovered from these samples. Two grams of UGL-33 rock were extracted to give 10.5 mg of extract, of which only 0.1 mg were saturated hydrocarbons and 0.7 mg were aromatic hydrocarbons. Eight grams of UGL-57 were extracted to give 12.5 mg of extract, of which 0.5 mg were saturated and 0.3 mg were aromatic hydrocarbons. The extremely small amounts of hydrocarbons obtained from these samples make them sensitive to contamination. The biomarker distributions obtained from UGL-57 (Fig. 18)

(A) Forillon Formation

25 , [] FF-orange

C • FF-dull

= o- 20-] ii~!~~i~it 2 15-I Cant hem domain

1 0 4

s4 all-liquid FIs

01 , , ,

0 50 100 150 Homogenization Temperature (°C)

(B) Indian Cove Formation

25

20

15

10

200

- / ' : ~ - ~ L 0 FF-zoned / - - ~ I '%-- [] FF-orange " ~ " O i [] I " ~ % • FF-dull

\ ~, I N N \ ' ~ ~ ~Southem domain \ \ I ~ .

Northern ~ ,t ~ % domain \ ' ~ ! II ~ .

. . . . . " _ , L~..__.~ ~ ~ / \ 0 ~ Centra domain

all-liquid F,, s \e~J , ,

50 100 150 200 Homogenization Temperature (°C)

Fig. 12. Histograms of homogenization temperatures of aqueous inclusions from the (A) Forillon and (B) Indian Cove formations. FI = fluid inclusion.

Fig. 13. Homogenization temperature and salinity crossplots for fluid inclusions from the (A) Forillon and (B) Indian Cove formations. FI = fluid inclusion.


Table 2. M i c r o t h e r m o m e t r i c r esu l t s o f f l u i d i n c l u s i o n s .

Sample Locality* Host Occurrence Size Type Tin. (°C) Salinity (wt % NaCl equiv.) Th (°C) mineral (ram) Range Mean (n) Range Mean (n) Range Mean (n)

Forillon Formation UGL-16 MA FF-orange Isolated 10 Aq -7.5 -7,5 (1) 11,1 11.1 (1) all liquid all liquid

Isolated 7 Aq -6.3 -6.3 (1) 9.6 9.6 (1) all liquid all liquid Isolated 6 Aq -7.2 -7.2 (1) 10.8 10.8 (1) all liquid all liquid Isolated 18 Aq -7.0 -7.0 (1) 10.5 10.5 (1) 40.5 40.5 (1) Cluster 11 Aq -7.6 -7.6 (1) 11.3 11.3 (1) 55.8 55.8 (1)

7 Aq -7.2 -7.2 (1) 10.8 10.8 (1) all liquid all liquid Isolated 15 Aq -7.2 -7.2 (1) 10.8 10.8 (1) all liquid all liquid Cluster 10 - 12 Aq -7.6 -7.6 (1) 11.3 11.3 (1) 61.7 ~ 62.8 62.3 (2)

UGL-19 MA FF-orange Growth zone 15 Aq -3.5 -3.5 (1) 5.7 5.7 (1) 48.5 48.5 (1) 8 Aq -3.8 -3.8 (1) 6.2 6.2 (1) all liquid all liquid

Isolated 18 Aq -2.5 -2.5 (1) 4.2 4.2 (1) 71.7 71.7 (1) Isolated 12 Aq 62.4 62.4 (1) Isolated 19 Aq -2.7 -2.7 (1) 4.5 4.5 (1) 73.4 73.4 (1) Cluster 6 ~ 8 Aq -3.8 -3.8 (1) 6.2 6.2 (1) 61.9 ~ 72.4 66.6 (3) Isolated 9 Aq 63.1 63.1 (1)

UGL-21 MA FF-orange Random 6 - 10 Aq -2.2 ~ -3.5 -2.9 (2) 3.7 ~ 5.7 4.7 (2) 58.8 - 63.4 61.6 (3) Random 5 ~ 12 Aq -5.3 -5.3 (1) 8.3 8.3 (1) 50.2 ~ 57.8 53.6 (3) Isolated 14 Aq -4.4 -4.4 (1) 7.0 7.0 (1) 62.5 62.5 (1) Isolated 8 Aq -5.5 -5.5 (1) 8.6 8.6 (1) all liquid all liquid Isolated 12 Aq -4.9 -4.9 (1) 7.7 7.7 (1) all liquid all liquid

UGL-39 GR FF-orange Random 5 ~ 9 Aq -11.8 ~ -15.8 Random 8 - 9 Aq -16.9 -16.9 (1) 20.1 20.1 (1) 66.2 - 72.2 69.2 (2)

UGL-48 RSJ FF-dull Random 4 ~ 5 Aq -3.0 ~ -3.8 -3.4 (2) 4.9 ~ 6.2 5.6 (2) 79.3 ~ 91.7 85.5 (2) Random 5 ~ 9 Aq -2.6 ~ -2.8 -2.7 (2) 4.3 ~ 4.6 4.5 (2) 88.4 ~ 103.0 95.3 (3) Random 5 Aq -3.6 -3.6 (1) 5.9 5.9 (1) 105.4 105.4 (1) Isolated 7 Aq 83.4 83.4 (1) Isolated 5 Aq -3.5 -3.5 (1) 5.7 5.7 (1) 109.7 109.7 (1)

Indian Cove Formation UGL-3 FP FF-Zoned Cluster 4 ~ 9 Aq -1,1 ~ -1.3 -1.2 (2) 1.9 ~ 2,2 2.1 (2) 55,6 ~ 77.1 65.0 (4)

Cluster 7 Aq -16,2 -16,2 (1) 19,6 19,6 ( t ) all liquid all liquid Isolated 6 Aq -20,0 -20,0 (1) 22.4 22,4 (1) 53.2 53,2 (1) Isolated 10 Aq -19,2 -19,2 (2) 21.8 21.6 (1) 66.2 66.2 (1) Cluster 8 ~ 25 Aq 6 9 . 0 - 80.4 74.1 (3) Cluster 12 ~ 15 Aq -0,1 ~ -0.2 0.2 (2) 0.2 ~ 0,4 0.3 (2) 65.0 ~ 67,6 66.5 (3) Isolated 11 Aq -22.0 -22.0 (1) 23.7 23.7 (1) 84.3 84.3 (1) Isolated 15 Aq -22.3 -22.3 (1) 23.9 23.9 (1) 55.6 55.6 (1) Isolated 7 Aq 67.4 67.4 (1) Isolated 7 Aq 66.7 66.7 (1) Isolated 15 Aq -17.0 -17.0 (1) 20.2 20.2 (1) all liquid all liquid

UGL-33 MA FF-dull Isolated 9 Oil 41.8 41.8 (1) Isolated 8 Oil 46.7 46.7 (1) Isolated 4 Oil 47.8 47.8 (1) Isolated 13 Oil 52.7 52.7 (1) Isolated 4 Oil 49.0 49.0 (1) Isolated 4 Oil 48.3 48.3 (1) Isolated 11 Oil 63.1 63.1 (1) Isolated 11 Oil 47.2 47.2 (1) Isolated 8 Oil 55.6 55.6 (1) Cluster 7 ~ 9 Oil 37 .6 - 50.3 44.0 (2)

UGL-36 BC FF-orange Random 8 ~ 15 Aq -9.4 ~-17.2 -13.3 (2) 13.3 ~ 20.4 16.9 (2) 68.1 - 98.7 79.0 (5) Random 8 ~ 20 Aq -14.2 - -18.2 -15.7 (3) 18.0 - 21.1 19.2 (3) 63.7 - 80.2 69.7 (3) Random 7 ~ 15 Aq -20.5 - -21.7 -21.1 (2) 22.7 ~ 23.5 23.1 (2) 74.4 ~ 86.4 82.2 (4) Random 6 - 9 Aq -16.6 ~ -17.3 -17.0 (2) 19.9 - 20.4 20.2 (2) 64.4 - 87.9 76.0 (4)

UGL-56 OC FF-orange Random 7 Aq -6.4 -6.4 (1) 9.8 9.8 (1) 74.2 74.2 (1) Random 9 - 12 Aq -11.7 - -20.7 -16.2 (2) 15.7 - 22.8 19.3 (2) all liquid all liquid Random 6 Aq 60.3 60.3 (1) Random 7 - 9 Aq 59.6 - 60.8 60.2 (2) Random 4 - 5 Aq 50.2 ~ 53.5 51.9 (2) Random 8 ~ 8 Aq -4.1 -4.1 (1) 6.6 6.6 (1) 67.2 - 68.7 68.0 (2)

UGL-57 OC FF-dull Isolated 10 Oil 61.5 61.5 (1) Isolated 8 Oil 56.0 56.0 (1) Isolated 5 Oil 65.8 65.8 (1) Isolated 8 Oil 68.3 68.3 (1) Isolated 9 Aq -3.9 -3.9 (1) 6.3 6.3 (1) 163.3 163.3 (1) Isolated 8 Aq 169.2 169.2 (1) Isolated 10 Aq -4.3 -4.3 (1) 6.9 6.9 (1) 154.8 154.8 (1)

* MA = Mississippi Anticline. GR = Grande Rivi6re. RSJ = Rivi6re St-Jean Anticline. FP = Forillon Peninsula. BC = Bazire Creek. OC = Oatcake Creek.

THE LOWER DEVONIAN UPPER GASPE, LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 359

Fig. 14. Oil inclusions in FF-dull calcite from samples (A) UGL-57 and (B) UGL-33.

indicate a low maturity marine source that is younger than Devonian, i.e. it has a relatively high abundance of C3o 4- desmethylsteranes, relatively high C28/C29 sterane ratio and sterane isomer ratios that have not reached, or are near, their equilibrium values as would be expected for an oil (Peters and Moldowan, 1993). It is very unlikely that hydrocarbons with these characteristics would be generated from this area, consid- ering the geology. Hence, this suggests that the small amount of hydrocarbons recovered from extraction of UGL-57 are not indigenous. Laboratory contamination was confirmed by a blank run at the same time which, similar to UGL-57, shows a gas chromatogram dominated by C22-C32 n-alkanes with a max- imum at C26 (Fig. 16) and similar biomarker distributions. Therefore, the results obtained from this sample were not considered to be representative of the hydrocarbons within the inclusions.

The data from UGL-33 are quite different from UGL-57 and the blank. The gas chromatogram shows a greater abundance of lighter n-alkanes (Fig. 16) and the biomarker distributions are closer to what would be expected of an oil derived from Devonian or older source rocks. Moreover, the biomarker distributions show many similarities to an oil (Galt No.I) produced from Upper Gasp6 Limestones in the same area (Fig. 18). These characteristics include a C27-C29 sterane distribution, which shows a large predominance of C29 over C27 and C28 steranes, high abundance of diasteranes relative to regular steranes, low abundance of homohopanes relative to C29 and C30 hopanes, relatively high abundance of tficyclic terpanes compared to hopanes and similar amounts of the C24 tetracyclic terpane to the C26 tricyclic terpane. There are some differences

6

5 2

0 50 100

UGL-33 [ ] Oil inclusion

[ ] Aqueous inclusion

I . . . . I

150 200

i1 UGL-57 [] Oil inclusion [] Aqueous inclusion

nH . . . . nH 50 100 150 200


Fig. 15. Histograms of homogenization temperatures of aqueous and oil inclusions in FF-dull calcite from samples UGL-33 and UGL-57. FI = fluid inclusion.

between UGL-33 and the oil, such as the lower relative abundance of short-chain steranes and rearranged hopanes (e.g. Ts/Tm ratio lower in UGL-33), which are consistent with a lower maturity for the oil inclusion hydrocarbons compared to the crude oil. There are also additional contamination peaks that elute around the retention times of the C33-C35 homohopanes in


the m/z 191 fragmentogram of UGL-33, which were present in the blank, and a low amount of C30 4-desmethylsteranes, which GC-MS analysis has indicated are not present in the Gait No. 1 oil but are in high abundance in the blank. This suggests that there is a small amount of contamination of this sample, which is not surprising, given the very low amount of extractable hydrocarbons. Despite this, based on this data it is probable that the hydrocarbons in the UGL-33 fluid inclusions have the same source as that of the Gait No. 1 oil. From the characteristics of the oil, the source of these hydrocarbons is a Devonian, or older, black marine shale.

DIAGENETIC EVOLUTION AND IMPLICATIONS FOR RESERVOIR POTENTIAL

Various diagenetic tracers and petrographic observations are used to interpret the diagenetic history for the Upper Gasp6 Limestones. The porosity history (formation and occlusion) according to depth is discussed from the perspective of hydrocarbon migration. The main distinctions in the geochemical

tracers are related mostly to the geographical (and paleogeographical) location or samples, but stratigraphic position (formations) of the samples also has some control.

N o r t h e r n D o m a i n

The study of the succession north of the Bassin Nord-Ouest Fault focuses on the evolution of both primary and secondary porosity in brachiopod-rich facies of the Indian Cove Formation. In areas west of the Forillon Peninsula, primary porosity is almost absent. Some fracture-filling calcite samples were collected in order to document any links between fluid cir- culation in fractures and late primary pore space enlargement in this area. The diagenetic evolution of the Indian Cove Formation in this area is illustrated in Figure 19.

Intra-brachiopod cements are typified by an early cement phase (PFNL) and stable isotope values of 8180 = -3.2%0 and 813C = +2.1%o, similar to Lower Devonian marine calcite (~180 = -2.7%~ and 813C = +1.5%e; Lavoie, 1993). The following strongly zoned (PF-zoned) and dull luminescent (PF-dull)

8 ° 0 - -

7 . 5 - -

>

7 . 0 - o

o ~- 6 . 5 -

6 . 0 - -

5.5

5

8 . 0 - -

7 . 5 - -

> E 7 . 0 - -

8 C

~ 6 . 5 -

6 . 0 -

I lO

c: !c c 1 8

C 1 6

I I 15 20 25

C 2 4

C 2 6

C 2 8

ii 3o

U G L - 3 3

I I I I I I I I 30 35 40 45 50 55 60 65

T i m e ( m i n )

C 2 4 C 2 6

C 2 2 C 2 8

C 2 0 C 1 8 .

C 1 6 /

I I 15 20 25

C 3 0

U G L - 5 7

I 3o

5.5 I I I I I I 5 10 35 40 45 50 55 60

T i m e (min)

I 65

Fig. 16. Saturate fraction gas chromatograms of oil inclusions from samples UGL-33 and UGL-57.

THE LOWER DEVONIAN UPPER GASPt~ LIMESTONES." CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL 361

calcite cements are both characterized by low 8180 values (-9.2 to -11.1%o) and relatively low (compared to fracture-fill calcite cement elsewhere) 813C values (+0.7 to +1.3%o). It is proposed that these last two cements were precipitated from meteoric water. This interpretation is supported by the stable isotopic and fluid inclusion data of the fracture-filling zoned calcite cement (FF-zoned) crosscutting the lithofacies, the former being characterized by very low 8180 (-11.2 to -12.4%o) and 813C (-1.5 to +0.3%0) values, and by the presence of fluid inclusions with very low salinity values (0.3 wt % NaCl-equivalent). It is likely that this late meteoric event, related to the lrnid-Devonian Acadian regional exposure (Bourque et al., 2001, this issue), was responsible for the local enhancement of the primary porosity of the brachiopod beds, although its exact timing could not be ascertained (Fig. 19). No evidence for hydrocarbon migration was found in the Indian Cove Formation samples of the northern domain.

CENTRAL AND SOUTHERN DOMAINS

The central and southern domains include distal outer shelf and toe-of-slope settings (Lavoie, 1992a). For these domains, the fracture-filling calcite cements have been shown to be rather uniform petrographically. A first fracture event was almost coeval with the inception of compaction stylolites. The fractures were filled by orange-dull calcite cement (FF-orange). The ensuing fracture event was accompanied by some dissolution of previous fracture-filling calcite cements, and post-dates bedding-parallel stylolites. Open voids associated with this fracture event (both fracture and dissolution voids) were ahnost totally occluded by a dull calcite cement (FF-dull). Finally, a poorly expressed third fracture set, also associated with dissolution of previous cements, occurred before the formation of bedding-perpendicular stylolites. The voids associated with this fracture event were also virtually occluded by a bright luminescent calcite cement (FF-bright). This general pattern is consistent for all studied localities and only differs in the intensity of the various fracture events. However, major differences in geochemical characteristics of cements are obvious from one locality to the other, especially between the central and southern domains.

Central Domain Samples included in this area are those from the Mississippi

Anticline and from the Galt and Sunny Bank cores (Fig. 1). The diagenetic evolution of the limestone in this area is illustrated in Figure 20. The first two fracture-filling cements (FF-orange and FF-dull) consist of calcite derived from saline, marine- derived brines precipitated at relatively low temperature (Th of aqueous inclusions <84°C; see Table 2 and Fig. 12). These low temperatures are consistent with relatively high 8180 values (Table 1, Figs. 9, 11) and suggest relatively shallow burial conditions. It is noteworthy that dissolution of calcite cements is obvious within fractures. This suggests the migration of aggres- sive C02-1oaded brines in the fracture system before cementation, a phenomenon common to hydrocarbon-bearing systems

(Feazel and Schatzinger, 1985). However, the 813C values of the cements are generally comparable to that of the inferred Early Devonian sea water (Lavoie, 1993), indicating that a marine-derived bicarbonate reservoir is dominant for CaCO 3 cementation. The occurrence of oil inclusions in FF-dull cement suggests that pulses of hydrocarbons occurred in response to the fracturing events. Estimated fluid pressure (341.5 bars) suggests a burial depth of around 1500 m for a lithostatic pressure system (burden density is assumed to be 2.3g/cm3). As discussed previously, the hydrocarbons within the oil inclusions of UGL-33 appear to have the same source as oil produced in the same area. However, the oil-source rock correlation needs further investigation.

600

500 -

4oo

300-

2 0 0

100

0

UGL-33

00 0 100 200 300 400 Temperature (°C)

1000

800

.~ 600

400

200

0 -100

UGL-57

....................... I I ~ [ --Trapping pressure I (763.t bars)

,,,I r P

~ T ~ o i l ~ ~ ....... I

I ] , 0 100 200 3oo 4oo s

Temperature (°C)

500

500

Fig. 17. Bubble-point curves and isochores of oil inclusions from FF- dull calcite of samples UGL-33 and UGL-57. The trapping temperature is assumed to be equal to the homogenization temperature of aqueous inclusion, and the trapping pressure is estimated from the isochore at the trapping temperature. The bubble-point curves and isochores were constructed using the VTFLINC program of Calsep A/S, with the oil composition being modelled by mixing 66% black oil (API= 34.3) and 34% volatile oil (API=50.1) for sample UGL-33 (AP1=39.7), and by mixing 57% black oil and 43% volatile oil for sample UGL-57 (API=41.1). pr = pristane, ph = phytane.

362 D. LAVOIE, G, CHI and M. FOWLER

Southern Domain

Samples included in this domain are those from the Grande Rivi~re, Rivi~re Saint-Jean Anticline, Bazire Creek and Oatcake Creek areas (Fig. 1). The diagenetic evolution in this domain is illustrated in Figure 21. The fracture-filling calcite cements (FF-orange and FF-dull) were precipitated from saline, marine-derived brines, but temperatures of precipitation were slightly (Grande Rivi~re) to significantly (Bazire and Oatcake creeks) higher than cements in the same formations in the Mississippi Anticline area. This increase in fluid temperature is reflected both by the 5180 values (Figs. 9, 11) and the homogenization temperatures of fluid inclusions (Fig. 12). Dissolution of fracture-filling calcite cements is present and occurred repet- itively, although open voids were less commonly preserved at

these localities. The occurrence of oil inclusions in FF-dull cement in the Oatcake Creek area suggests that pulses of hydrocarbons occurred in response to the fracture events. Estimated fluid pressure (763.1 bars) suggests a burial depth of around 3380 m for a lithostatic pressure system (burden density is assumed to be 2.3g/cm3), which is significantly deeper than the Mississippi Anticline area.

IMPLICATIONS FOR PETROLEUM RESERVOIR POTENTIAL

The fluid inclusion study indicates that liquid hydrocarbons were present during cementation of some fracture systems in the Upper Gasp6 Limestones. Therefore, some fractures were open when liquid hydrocarbon flowed trough the strata. If suit- able trapping conditions existed, the fractured Upper Gasp6

U G L - 5 7 - oi l i n c l u s i o n C U G L - 5 7 - o i l i n c l u s i o n rrgz 191 - distr ibut ion o f te rpanes m/z 217 - distr ibut ion o f steranes

30 J 21 29

33 27D. 31

32

23td 29

Tm

24tet Ts i " ' ~ ~ / ~ / 1 1 ~ , l , l ~ / t ~ J ~ J 26tri j

U G L - 3 3 - o i l I n c l u s i o n 3 rrVz 19 f - distr ibut ion o f terpanes

29

23td

J l 24tet T2~

Gai t -1 - p r o d u c e d oi l m/z 191 - distr ibut ion of terpanes

30

C

U G L - 3 3 - oi l i n c l u s i o n m/z 2 1 7 - distr ibut ion o f steranes

27D

21

Gai t -1 - p r o d u c e d ol l rn/z 217 - distr ibut ion o f steranes

21

23tri

• Ts 29

29D 29 J 28

29

29D

U G L - 5 7 - oi l i n c l u s i o n m/z 218 - distr ibut ion o f steranes

21

27

29

U G L - 3 3 - o i l i n c l u s i o n rrgz 218 - distr ibut ion o f steranes

29

21

Gai t -1 - p r o d u c e d oi l m/z 218 - distr ibut ion o f steranes

29

27

Fig. 18. m/z 191, m/z 217 and m/z 218 fragmentograms of oil inclusions from samples UGL-33 and UGL-57 and of produced oil f rom the Gait No. 1 well. Ts = total sterane, T m = total macerals, tri = tricyclic, tet = tetracyclic.

THE LOWER DEVONIAN UPPER GASPt~ LIMESTONES: CARBONATE D1AGENESIS AND RESERVOIR POTENTIAL 363

Limestones may be potential petroleum reservoirs. Salinity of aqueous inclusion data indicate that oxidizing meteoric water was not involved in the cementation, except in a few fractures in the northern domain. There may not have been significant heating after fracture cementation; therefore, if a petroleum reservoir was formed, higher burial temperatures and reservoir destruction would not be expected.

CONCLUSIONS The Upper Gasp6 Limestones contain hydrocarbon seeps

and reservoirs with limited liquid and gas hydrocarbon production. Two main questions are addressed in our study: (1) what is the regional significance of the primary pore space preserved in the Indian Cove Formation on Fofillon Peninsula? and (2) what is the regional significance of the fracture-restricted secondary porosity known to be present in the Forillon and Indian Cove formations in central Gasp6? Our study demonstrates that the dissolution-enlarged primary pore space present in the brachiopod-rich beds of the Indian Cove Formation on Forillon Peninsula (Lavoie, 1996b) does not extend westward. The diagenetic study of the pore- and fracture-filling calcite at these sites indicates that a late (post-Acadian) meteoric influx occurred and was responsible for dissolution enlargement of the pore space in the lithofacies.

A significant secondary porosity was documented in all three formations of the Upper Gasp6 Limestones in central Gaspr; the samples from the Indian Cove Formation contain a greater volume of preserved open pore space than the other formations. It is noteworthy that, regionally, the Mississippi Anticline (in north-central Gaspr) is not only the site of most hydrocarbon seeps, but also the area from which secondary pore space is most abundant. This particular area has a low maturation level (Bertrand, 1996; Bertrand and Malo, 2001, this issue). Our study has documented the presence of abundant oil inclusions in locally, still open, fracture-filling calcites.

EVENTS

Sedimentation

Compaction

Pressure solution

First fractures

Second fractures

Otasolufton

Third fractures

Dtasolution

Fracture-tilling calcite

Uplift [ ]

S e c o n d a r y P o

UGL - C E N T R A L DOMAIN MISSISSIPPI ANTICLINE

TIME -~,

MARINE BURIAL Deep

i i l i i i i m l l m i m

m

iml

FF-or. FF-duII FF-brl. I I l I I I l l l m

I ~.~.mm~,~.,)

POST. BURIAL

Fig. 20. Schematic diagenetic evolution of the Upper Gasp~ Limestones in the distal outer shelf (central) domain. FF = fracture-fill, or. = orange, bri. = bright, Hc = hydrocarbons.

Moreover, this area experienced a lower thermal history compared to adjacent areas in the south and the west (Bertrand, 1987; Bertrand et al., 1992; Bertrand and Malo, 2001, this issue). Samples from localities south, southeast, and west of the Mississippi Anticline are characterized by a lower percentage of fracture porosity. The various diagenetic tracers indicate that thermal conditions were slightly to significantly higher in these areas than in the Mississippi Anticline. However, the potential of these areas for hydrocarbon reservoirs cannot be ruled out, because abundant oil inclusions occur in fracture-filling calcite cements. The westward, general increase in thermal conditions has been reported previously (Belrtrand et al., 1992; Bertrand,

EVENTS

Sedimentation

Cementation

Compaction

Pressure solution

Sudal fractures (not studied)

Uplift [ ]

Meteoric fractures + cementation

Primary Pc Secondary Pc

UGL - NORTHERN DOMAIN TIME '~.

MARINE I BURIAL Shallow I Interm. I Deep

l PF-NL I l i i m

? ?

5

practures ? ~ ? Meteoric

POST- BURIAL

FF lind PF-zoned

Fig. 19. Schematic diagenetic evolution of the Indian Cove Formation (Upper Gasp~ Limestones) in the proximal outer shelf (northern) domain. FF = fracture-fill, PF = pore-fill.

UGL - S O U T H E R N D O M A I N GRANDE RIVI~RE I BAZlRE I OATCAKE

TIME

EVENTS

Sedimentation

Compaction

Pressure solution

First fractures

Second fractures

Dissolution

Third fractures

Dissolution

Fm©ture..fll,ng calcites

Uplift [ ]

Secondary Pc

BURIAL Shallow I Interm. I <tDeeP70°C

m l

l

l

FFm~r. FF-dull F~-brl s i i I I I

Fig. 21. Schematic diagenetic evolution of the Upper Gaspe Limestones in the southern domain and westernmost sections (Grande Rivi~re, Rivi~re St-Jean, Bazire Creek and Oatcake Creek). FF = fracture-fill, or. = orange, bri. = bright, Hc = hydrocarbons.

364 D, LAVOIE, G. CH1 and M. FOWLER

1996), but the southward increase, even though suggested from independent stratigraphic data, has not been documented previously from organic matter data.

The data generated by this study indicate that the Upper Gasp6 Limestone succession in north-central Gasp6 (e.g. Mississippi Anticline area) has a good potential for fracture reservoirs. This conclusion is supported by regional maturation studies and the local known occurrences of hydrocarbon seeps and limited production. The Indian Cove Formation in areas south and west of the Mississippi Anticline area is apparently also a good candidate for hydrocarbon reservoirs. Therefore, detailed structural studies are needed in order to delineate fractured subsurface traps, in particular in areas characterized by high density of crosscutting faults.

A C K N O W L E D G M E N T S

Special thanks are directed to Rudolf Bertrand for helpful discussions and for providing geochemical data of produced oil, and to Lavern Stasiuk for some fluorescence spectrometric measurements of oil inclusions. A first draft of the paper bene- fitted from a detailed review by Norman Tassr, and the final draft was improved through the input of Pierre-Andr6 Bourque. CSPG reviewers, John Weissenberger and Ihsan A1-Aasm, provided excellent comments and suggestions that were much appreciated, and helped to improve the paper. We would like to acknowledge Shell Canada's financial support for the research and its publication, and the involvement of their entire eastern Canada Frontier basin staff. The Geological Survey of Canada is also thanked for supporting some analyses through its A-base and Appalachian Foreland and Platform (NATMAP) funding. This is Geological Survey of Canada Contribution No. 200088.

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