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Bulletin of Earth Sciences of Thailand 115 Riadi, Depositional Environments and Stratigraphic Development of the Grand Taman Sari Circuit Outcrop: an Analogue for Transgressive Mahakam Delta Successions. Vol. 6, No. 2, pp. 115-121 Depositional Environments and Stratigraphic Development of the Grand Taman Sari Circuit Outcrop: an Analogue for Transgressive Mahakam Delta Successions Ridha Santika Riadi Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok10330, ailand Corresponding author email: [email protected] Abstract A detailed analysis of the sedimentology and stratigraphy of the Grand Taman Sari Circuit outcrop indicates that there were tide-dominant, transgressive deltaic successions with highly retrogradational parasequence sets in the middle Miocene Mahakam Delta province. A 115 meter thick fluvial succession is overlain by an increasingly marine-upward, 280 meter thick transgressive succession. ere are two sandstone facies that have reservoir potential. Fluvial channel sandstones up to five meters thick and less than three hundred meters wide are laterally discon- tinuous, occasionally stacked and difficult to correlate while back-filled distribu- tary channel sandstones are up to twenty meters thick and more than five hundred meters wide. It is difficult to distinguish between the two sandstone facies in the subsurface because they have similar fining upward log signatures. Keywords: Transgressive, Fluvial system, Shallow Marine system, back-filled distributary channel 1. Introduction e Mahakam Delta is regarded as one of the world’s best examples of a mixed river-dominated and tide-dom- inated delta based on its morphology (Galloway, 1975). e traditional view suggests that the modern Mahakam Delta is a progradational deltaic sys- tem and that the middle Miocene of the Mahakam Delta province was ex- clusively progradational deltaic system (e.g. Allen et al. 1998, Trevena 2003, and Cibaj 2008). In contrast, a recent study suggests that the modern Mahakam Delta is being transgressed, is strongly tide-dom- inated and its morphology is a relic of fluvial morphology that has been partially modified by tidal processes (Salahuddin and Lambiase 2013). Sim- ilarly, Nirsal (2010) interpreted a mid- dle Miocene Mahakam Delta outcrop as transgressive and Lambiase et al. (2010) identified transgressive succes- sions in core and on seismic data, in- dicating that transgressive successions are not restricted to the modern delta. e Grand Taman Sari Circuit outcrop, located in Samarinda province (Fig. 1),
Transcript of Bulletin of Earth Sciences of Thailand Santi… · a younger shallow marine deposition-al...
Bulletin of Earth Sciences of Thailand
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Riadi, Depositional Environments and Stratigraphic Development of the Grand Taman Sari Circuit Outcrop: an Analogue for Transgressive Mahakam Delta Successions.
Vol. 6, No. 2, pp. 115-121
Depositional Environments and Stratigraphic Development of the Grand Taman Sari Circuit Outcrop: an Analogue for Transgressive Mahakam
Delta Successions
Ridha Santika Riadi
Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok10330, Thailand
Corresponding author email: [email protected]
Abstract
A detailed analysis of the sedimentology and stratigraphy of the Grand Taman Sari Circuit outcrop indicates that there were tide-dominant, transgressive deltaic successions with highly retrogradational parasequence sets in the middle Miocene Mahakam Delta province. A 115 meter thick fluvial succession is overlain by an increasingly marine-upward, 280 meter thick transgressive succession. There are two sandstone facies that have reservoir potential. Fluvial channel sandstones up to five meters thick and less than three hundred meters wide are laterally discon-tinuous, occasionally stacked and difficult to correlate while back-filled distribu-tary channel sandstones are up to twenty meters thick and more than five hundred meters wide. It is difficult to distinguish between the two sandstone facies in the subsurface because they have similar fining upward log signatures.
Keywords: Transgressive, Fluvial system, Shallow Marine system, back-filled distributary channel
1. Introduction
The Mahakam Delta is regarded as one of the world’s best examples of a mixed river-dominated and tide-dom-inated delta based on its morphology (Galloway, 1975). The traditional view suggests that the modern Mahakam Delta is a progradational deltaic sys-tem and that the middle Miocene of the Mahakam Delta province was ex-clusively progradational deltaic system (e.g. Allen et al. 1998, Trevena 2003, and Cibaj 2008).
In contrast, a recent study suggests that the modern Mahakam Delta is being transgressed, is strongly tide-dom-inated and its morphology is a relic of fluvial morphology that has been partially modified by tidal processes (Salahuddin and Lambiase 2013). Sim-ilarly, Nirsal (2010) interpreted a mid-dle Miocene Mahakam Delta outcrop as transgressive and Lambiase et al. (2010) identified transgressive succes-sions in core and on seismic data, in-dicating that transgressive successions are not restricted to the modern delta. The Grand Taman Sari Circuit outcrop, located in Samarinda province (Fig. 1),
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offers more understanding about the ancient Mahakam Delta; this study fo-cuses on its stratigraphic architecture and depositional environment.
2. Sedimentary Facies and Depositional Environments
Thirteen facies were defined from sedimentary structures, bed thickness and contacts, bioturbation and grain size, sorting and spherici-ty, plus 25 microfossil and palynology analyses. They include:
• Non-marine mudstone • Conglomeratic pebbly sand-stone • Interbedded sandstone and non-marine mudstone • Stacked medium sandstones • Thin bedded coal • Cross bedded coarse sand-stone • Shallow marine mudstone • Alternating sandstone and shallow marine mudstone • Bioturbated medium sand-stone • Cross bedded medium sand-stone • Alternating HCS sandstone and shallow marine mudstone • Bioturbated muddy sandstone • Interbedded bioturbated fine sandstone and shallow marine mud-stone
Four facies were deposited by a meandering fluvial system. The non-marine mudstone facies is flood-plain deposits and the conglomeratic pebbly sandstones are fluvial channel
deposits. Crevasse splays deposited the interbedded sandstone and non-ma-rine mudstone and stacked medium sandstone facies. The other nine facies are associ-ated with marine and marginal marine depositional environments. Cross-bed-ded coarse sandstone facies represent a back-filled distributary channel and, based on palynology, the thin-bedded coals are mangrove swamp deposits. The remaining facies were deposited in more strongly marine-influenced en-vironments, including tidal sand flats (the alternating sandstone and shallow marine mudstone plus the alternating HCS sandstone and shallow marine mudstone), tidal sand bars (bioturbat-ed medium sandstone and bioturbat-ed muddy sandstone), tidal mud flats (shallow marine mudstone), tidal chan-nels (cross-bedded medium sandstone) and shoreface (the interbedded biotur-bated fine sandstone and the shallow marine mudstone facies).
3. Stratigraphic Architecture
Generally, this outcrop can be divided into an older fluvial system and a younger shallow marine deposition-al environment. Facies correlation was difficult in the fluvial system because the fluvial channels are relatively nar-row (Fig. 2). Generally, the fluvial sys-tem is a progradational parasequence set where the number of parasequences varies laterally from two to three (Fig. 2). The boundary between the flu-vial system and shallow marine deposi-tional environments is marked by a ma-jor erosive surface which is a possible
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sequence boundary. The surface is the base of a channel that cut into the older fluvial sediments during a lowstand; it is interpreted as a distributary because it is at least 500 meters wide, which is much larger than the fluvial channels, and is filled with marine sediment (Fig. 2). Relative sea level started to rise after the channel incision and abandonment. The distributary channel began to back-fill with mixed fluvial and tide deposits and, as transgression continued, it was converted into an estuary with muddi-er, finer, and more marine-influenced deposits. Facies in the shallow marine succession can be correlated laterally, including the back-filled distributary channel (Fig. 2). The vertical succes-sion consists of four progradational parasequences and three aggradation-al parasequences within highly retro-gradational parasequence sets (Fig. 2). Each progradational cycle varies from 17 to 58 meters thick and consists of a back-filled distributary channel or tid-al channel at the base followed by tidal flat or tidal bar sands with an overlying tidal mud flat or marginal marine coal (Fig. 2). Generally the progradational parasequences become thinner, mud-dier and more marine upward so that the whole succession is transgressive. The aggradational cycles vary in thick-ness from 13 to 17 meters and consist of tidal flat mud covered by marginal marine coal (Fig. 2).
4. Reservoir Potential
There are two facies with res-ervoir potential, fluvial channel sand-stones and back-filled distributary channel sandstones. Fluvial channel sandstones fine upward from poor-ly sorted coarse to medium sand and often have pebbly and mud clast con-glomeratic lag deposits at their base. They occur in lenticular beds that are up to 5 meters thick and less than 300 m wide based on channel width to sand thickness ratios (Gouw 2007). Conse-quently, they do not generate signifi-cant connected reservoir volume un-less several sands are vertically stacked. Back-filled distributary chan-nel sandstones are cross-bedded, fin-ing-upward coarse to fine sandstones with pebbly coal clasts and abun-dant flaser bedding. The amount of cross-bedding decreases upward while the amount of tidal influence increases upward. These sandstones are 8 to 21 meters thick, extend laterally for more than 500 meters in outcrop and could be up to 2 km wide using the modern Mahakam Delta as an analogue. There is no relationship between channel width and sandstone thickness because thickness depends only on the amount of back-filling before channel avulsion; therefore, thickness-width ratios are meaningless. Fluvial channel sandstones and back-filled distributary channel sand-stones are difficult to distinguish in the subsurface because both facies fine up-ward and are expected to generate sim-ilar log patterns (Fig. 3). In outcrop, the two types of sandstone occur within 40 meters of each other stratigraphically
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so one could easily be misinterpreted; detailed biostratigraphic analysis is the best way to distinguish them.
5. Conclusions
The sedimentology and strati-graphic architecture of the Grand Ta-man Sari Circuit outcrop indicates that: a.It was deposited in fluvial and shallow marine depositional environ-ments. b. A basal fluvial succession is separated from the overlying transgres-sive shallow marine succession by a lowstand erosive surface that is the base of a distributary channel and could be a
sequence boundary. c. Fluvial channel sandstones and back-filled distributary channel sandstones have excellent reservoir po-tential but they have significantly dif-ferent reservoir geometries, volumes and connectivity. d. The two types of reservoir sandstone are difficult to distinguish in the subsurface because they generate similar log signatures.
Figure 1. The location of the Grand Taman Sari Circuit outcrop. A) The general area in eastern Kalimantan, B) the outcrop location with respect to Samarinda City, and C) the exposed stratigraphic sections (white lines) near the race track (yellow line).
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Figure 3. A) The stratigraphic succession in a fluvial channel sandstone and its predicted gamma ray log pattern, B) the stratigraphic succession and gamma ray log pattern of a back-filled distributary channel sandstone.
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Acknowledgments
The author would like to thank Prof. Joseph Lambiase for his patient guidance and supervision during this research and for editing of this man-uscript. The author wants to thank PTTEP and Chevron Thailand for fi-nancial support and Dr. Mike Bidgood and Dr. David Shaw for the biostrati-graphic analyses.
References
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Galloway, W.E., 1975, Process framework for describing the morphologic and stratigraphic evolution of the deltaic depositional systems, in Broussard, M.L., ed., Deltas, Models for Exploration: Houston Geological Society, Houston, p. 87-98.
Gouw, M.J.P., 2007, Alluvial architecture of fluvio-deltaic successions : a review with special reference to Holocene Settings. Netherlands Journal of Geosciences. v. 86, p 211 - 227.
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Nirsal, N., 2010, Facies distribution and stratigraphic development in the paleo-Mahakam Delta, Indonesia: M.Sc. thesis, Chulalongkorn University, Bangkok, 83 p.
