Volume curvature attributes to identify subtle faults and fractures in carbonate reservoirs:

Cimarrona Formation, Middle Magdalena Valley Basin, Colombia. Luis Bravo* and Milagrosa Aldana, Simón Bolivar University, Caracas, Venezuela.

Summary

In this work we analyze a natural fractured carbonate reservoir in the Middle Magdalena Valley Basin (Colombia) using different volume curvature attributes. The main objective was to illuminate small or sub-seismic faults, and to delineate fractured zones in the area. Most positive, most negative, dip, strike, curvedness, Gaussian, maximum, minimum and mean curvature attributes were calculated for a 3D post-stack time migrated seismic

volume. To reduce the noise level, a Butterworth filter was applied previous to the curvature attributes extraction and a combination of median and mean filters later. The most positive and most negative attributes gave the best results. They allowed identifying discontinuities at the target horizon, illuminating short wavelength flexures and lineaments. These features were associated to fracture zones that could result in hydrocarbon accumulation areas.

Also, local geomorphological features and lineaments related to the structure axes were also identified using these attributes.

Introduction The analysis of post-stack seismic attributes, as seismic coherence (Bahorich, et al., 1995) and semblance (Marfurt,

et al., 1998), have been widely used to delineate faults and stratigraphic characters. They measure lateral variations in the waveform and amplitude. Attributes obtained from the first surface derivative, as dip and dip azimuth, can illuminate small faults with a displacement lower than the seismic wavelength (Brown, 2004). Nevertheless, these methods can delineate lineaments but they cannot distinguish between symmetric and asymmetric features (e.g. faults from ridges or valleys) (Roberts, 2001). Second

derivative or curvature attributes gave more insight regarding the delineation of faults with throw under seismic resolution, as well as they can help in the prediction of fractures and their orientation (Roberts, 2001; Chopra et al., 2007). These attributes have been correlated with open fractures in outcrops (Lisle, 1994), and production tests (Nissen et al., 2007). Bergbauer, et al. (2003) calculated curvature using different wavelengths filtering the input

horizon; they concluded that images at different wavelengths provide diverse perspectives of the same geology. Al-Dossary et al. (2006) extended the concept of surface curvature to volumetric curvature. This allows calculating curvature in seismic volumes avoiding the need of picking a horizon (Chopra et al., 2007).

In the present work, several volume curvature attributes are applied to a natural fractured carbonate reservoir trying to

identify subtle faults and fractures, difficult to observe in the seismic data or using other attributes. These attributes could be an important tool in the characterization and delineation of fractured structures at the Late Cretaceous of Middle Magdalena Basin, where the production is mainly associated to carbonate fractured reservoirs.

Theory

Curvature is a property that quantifies how much a curve deviates from a straight line at a point (Roberts, 2001). In two dimensions, it can be defined as the radio of the circle tangent to a curve. Consider a 2D section through a plane surface or horizon (see figure 1) whose normal vectors are drawn at regular intervals along this horizon. Where the corresponding vectors are all parallel, the curvature is cero.

If the horizon has the form on an anticline, the vectors diverge and the curvature is defined as positive. If the horizon is a synclinal, the vectors converge and the curvature can be defined as negative (Robert, 2001).

Figure 1: Curvature in two dimensions (from Roberts

(2001)).

The extension of this concept to three dimensions uses orthogonal planes to the surface to measure the curvature in each point of the horizon (Hart et al., 2007; Roberts, 2001). Figure 2 illustrates this concept. The intersection of two orthogonal planes at the surface describes the maximun (Kmax) and minimum (Kmin) curvature. Other orthogonal planes let define the normal curvature in the dip directions

(Kdip), perpendicular to this (Kstrike) and along the structural contours (Kcontour).

Positive Curvature

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Syncline

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Volume curvature to identify fractures

seg. 1.1

1.24

1.38

1.52

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Figure 2: Curvature in three dimensions (modified from

Hart et al., (2007) and Roberts (2001)). When curvature information is extracted, the sampling interval and hence the aperture size should be careful defined. According to Bergbauer, et al., (2003) the change of the width aperture allowed observing features of different wavelengths. Figure 3 schematizes the differences that can be observed when calculating the curvature using

distinct apertures. With a small aperture, the event in black can be illuminated; a wide aperture illuminates the event in red and an aperture between these two values illuminates the yellow one.

Figure 3: Effect of the aperture size (modified from Hart et al. (2007)).

Study area and Methodology To the south of the Middle Valley Magdalena Basin, the Cimarrona Formation represents a natural fractured reservoir mainly composed by fractured limestones and calcareous sandstones in a structurally complex and deformed area (Acosta, 2002; Mondragon et al., 2009). The

knowledge, characterization and mapping of the fracture systems are key factors for the development of this reservoir. A previous seismic interpretation of the area pointed to the existence of major faults in the zone. In fact they confirmed the presence of faults that cut the main structure of the field. Nevertheless, they could not give information regarding the length and direction of sub-seismic faults, i.e., those faults whose offsets do not

produce significant disruptions of the horizon. In order to test the performance of volume curvature attributes on the fractured carbonates of the Cimarrona Formation, a 3D post-stack time-migrated cube was analyzed. In this cube, the target horizon is represented by a high amplitude reflector, with a dominant frequency of

±24Hz. The seismic interpretation allowed mapping the structure of Cimarrona Formation. It consists of a deformed synclinal where a local anticline is also observed. The structure is cut by several fault systems. One has a dextral component in N45W direction; the other is inverse with an

approximate N-S direction (see figure 4).

Figure 4: Structural map of the study area (twt).

Results Semblance attributes were also calculated and extracted for

the seismic volume. In figure 5, the lineaments associated to the main faults that cut the structure (black arrows) can be observed. Nevertheless, the limitation of this attribute to define sub-seismic faults and fractured zones in the area is also illustrated. In fact, the fault that cut the structure to the north of the field is barely observed (blue arrow).

-

+

N

1 Km

Figure 5: Semblance slice at the Cimarrona Formation. Different volume curvature attributes (i.e. most positive, most negative, Mean, Gaussian, etc) were extracted. Also the size aperture was varied trying to get insight regarding

short wavelength events. A spatial aperture of 7x7 traces

High

Low

1 Km

N

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Volume curvature to identify fractures

and a temporal operator proportional to half the target wavelength, proved to be enough to effectively sampling the short wavelength events. To attenuate noise and high frequencies, a Butterworth 4-8Hz, 65-70Hz filter was applied to the data. With this filter and the application of

another after extracting curvature attributes, we tried to minimize to an acceptable level, those short wavelength events associated to seismic acquisition and processing. The curvature attributes that gave the best illumination of discontinuities and flexures at the Cimarrona Formation seemed to be the most positive, most negative and strike curvature. To correlate this lineaments or flexures with

small offset faults or fractured zones, available petrophysical data (i.e. image wells, petrophysical evaluations and rose diagrams) were analyzed. Figure 6 shows an interpreted seismic section close to the well with the highest oil production in the field. The faults identified as F1 and F2 present a higher absolute value for the most negative curvature. This was expected as their offsets create a disruption in the reflector AA’. The interpreted Fp

flexure shows a lower absolute value; nevertheless, the petrophysical evaluation and image profile of the well close to this feature indicate high fracture density to the top of the Cimarrona Formation. Hence the identified lineament in the curvature map could be related to this fractured zone that is more liable for hydrocarbon accumulations. As can be observed in figure 7, the most positive and most negative curvature maps for the Cimarrona Formation

clearly allowed identifying the discontinuities associated to major faults in the area. Also numerous minor lineaments can be observed. Some of these flexures correspond to small faults that can be directly observed in the seismic data; nevertheless, its manual interpretation is difficult to do, due the high density of events and the ambiguity in the signal character.

If the existence of a relation between the flexures and the fractured zones is assumed, some of these lineaments could correspond to faults whose offsets are not wide enough to produce a disruption in the target seismic horizon. In order to correlate them, the most significant lineaments are

identified with arrows and numbers in the maps of figure 7 and in the seismic profiles of figure 8. In some cases, as in the AA’ profile of this figure, it can be easily observed the seismic event that generated the lineament in the curvature volume. In profiles AA’ and BB’, the faults that cut the main structure of the field are identified. In profile CC’ the identified lineaments in the curvature maps are not so evident and seem to be related with the small oscillations of

the signal observed in the seismic sections.

Conclusions The most positive and most negative attributes gave the best results in the identification of discontinuities in the study area. These attributes enhanced small wavelength lineaments of flexures related to fractured zones that are

more reliable for hydrocarbon accumulations. Local geomorphological features and lineaments related to the structure axis were also illuminated. Our results also indicate that to illuminate short wavelength features without aliasing, the length of the sampling operator should be lower or equal to the half wavelength of the target reflector and pointed to the need of an effective and careful noise reduction before and after curvature calculations.

Acknowledgments We would like to thank Pacific Rubiales Energy for providing the data. We also would like to thank Geomodeling Technology Corporation for providing the software used to calculate the attributes.

Figure 6: Relation between curvature attribute, faults, flexures and fractures in the vicinity of the well with the highest

hydrocarbon production in the study area. The map location is enclosed with a black square in figure 7a.

Pos Neg

A A’

F1

F2 Fp

GR

0 150

200m

50 m

s

F1: Fault 1, F2: Fault 2 Fp: Flexure or fault with throw under seismic resolution limit. Petrophysics evaluation provided for PRE, (2007).

6382

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Volume curvature

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IN

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RIO

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(FMI) Image log

A A’

F1 F2 Fp

N

500m

Qo > 1000 bbl/d

500< Qo < 1000 bbl/d

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Volume curvature to identify fractures

Figure 8: Seismic profiles AA’, BB’ and CC’ showing locations of lineaments indicated by arrows in figure 7.

Figure 7: Volume curvature at the Cimarrona Formation: (a) most negative; (b) most positive. The yellow arrow indicates flexures associated to faults that cut and define the field structure.

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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2010 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES

Acosta, J., 2002, Estructura, Tectonica y Modelos en 3D del Piedemonte Occidental de la Cordillera Oriental y del Valle Medio del Magdalena, Colombia : PhD Dissertation, London University.

Al-Dossary, S., and K. Marfurt, 2006, 3D Volumetric multispectral estimates of reflector curvature and rotation: Geophysics, 71, no. 5, P41–P51, doi:10.1190/1.2242449.

Bahorich, M., and S. Farmer, 1995, 3D Seismic Discontinuity for faults and Stratigraphics Features: The Coherence Cube: The Leading Edge, 14, no. 10, 1053–1058, doi:10.1190/1.1437077.

Bergbauer, S., T. Mukerji, and P. Hennings , 2003, Improving Curvature Analyses Of Deformed Horizons Using Scaledependent Filtering Techniques: AAPG Bulletin , 87, no. 8, 1255–1272, doi:10.1306/0319032001101.

Brown, A., 2004. Interpretation of Three-Dimensional Seismic Data: AAPG Memoir 42.

Chopra, S. & Marfurt, K., 2007, Seismic Attribute for Prospect Identification and Reservoir Characterization: SEG Geophysical Developments No 11.

Hart, B., and J. Sagan, 2007, Curvature for Visualization of Seismic Geomorphology, in R. Davies, H. Posamentier, L. Wood, and J. Cartwright, eds., Seismic Geomorphology: Applications To Hydrocarbon Exploration And Production: Geological Society, Special Publication, 139-149.

Lisle, R., 1994, Detection Of Zones Of Abnormal Strains In Structures Using Gaussian Curvature Analysis: AAPG Bulletin , 78, 1811–1819.

Marfurt, K., L. Kirlin , S. Farmer, and M. Bahorich, 1998, 3-D Seismic Attributes Using a Semblance-based Coherency Algorithm: Geophysics, 63, 1150–1165, doi:10.1190/1.1444415.

Mondragon, J., M. Mayorga, G. Rodriguez, J. Navarro, and I. Moretti, 2009, Nuevas Perspectivas Exploratorias en el Sector Sur de la Cuenca del Valle Medio del Magdalena Colombia : X Simposio Bolivariano de Exploración Petrolera en Cuencas Subandinas, Expanded abstract.

Nissen, S., T. Carr, and K. Marfurt, 2007, Using New 3-D Seismic Attributes to Identify Subtle Fracture Trends in Mid-Continent Mississippian Carbonate Reservoirs: Geophysical Society of Kansas, 3, no. 3, 9–12.

Pacific Rubiales Energy, PRE, 2007, Internal Report.

Roberts, A., 2001, Curvature Attributes And Their Application to 3D Interpreted Horizons : First Break, 19, no. 2, 85–100, doi:10.1046/j.0263-5046.2001.00142.x.

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