Understanding of the Heterogeneity in the Carbonate/Clastics Reservoirs

Compared to other types of reservoirs such as sandstone reservoirs, carbonate drilling is commonly more challenging due to intrinsic heterogeneities witnessed during both observation and measurement (Questiaux, Couples & Ruby, 2010). In relation to carbonate explorations, heterogeneity is attributed to pore types, variable lithology, sedimentary facies, and pore connectivity, all which describe geological processes related to carbonate production and formation as well as to subsequent changes during digenesis process (Morse, Arvidson & Luttge, 2007). One of the common type of topologies that affect the quality of carbonate exploration is karstification. The features of a karst may develop either through epigene or through hypogene. The main difference between the two processes is that while the former is relatively shallow, the latter is deep. These changes are due to the source of the dissolving fluid. The dissolving fluid during epigene karstification is of a meteoric origin, while the karstificatoin is controlled by a shallow groundwater drainage, which develops either at the exposed surface or due to the influence of the percolating meteoric water. The origin of the dissolving fluid in hypogene karstification, on the other hand, is not meteoric and typically enters underground water system, where it is recharged from below (Morse et al., 2007).

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The focus on the implications of karstification on heterogeneity of carbonate reservoirs is important because it provides an in-depth understanding of the carbonate reservoir drilling process. Experts in the oil and gas industry get to understand the fact that a karst can negatively affect the overall quality of a carbonate reservoir. For instance, dissolution and brecciation during karst formation can increase porosity and permeability, whereas fluid flow during post-karstification may block porosity and reduce permeability. All these changes may pose a challenge to the drilling process. The need to understand the implications of a karst on the heterogeneity of a carbonate reservoir has prompted several research efforts in the field. Lucia (2000); Kennedy (2002); Elkateb et al. (2003) and Garland et al. (2012) demonstrate an in-depth and specific understanding of the karst formations and carbonate heterogeneities. Much of the available studies combine outcrop, petrographic and geochemical analysis, which can constrain the style, distribution and origin of seismic-scale karst, thus providing an improved understanding of carbonate reservoir architecture and allowing for the development of a better drilling program.

However, in spite of their immense contribution, relatively few studies use seismic data to characterize the regional development of the seismic-scale karst features. It is a widely acknowledged fact that seismic data provides an accurate and more valuable assessment of the extent and scale of karstification due to its inclusion of a variety of variables that allow for mapping and characterization of large-scale features, which may not be achievable with the borehole data alone (Carr et al., n.d). In this project, I will use time-migrated 3-D seismic data to determine the distribution, scale and origin of karst in a Jurassic-Cretaceous carbonate dominated succession in the Jubah area. I chose to study Jubah because it is located in the margin between Gotnia and Central Arabian basins and that it has undergone at least one extended period of subaerial exposure-related karstification during the Late Cretaceous (Pre Aruma Unconformity). The goal of this study is to come up with a model showing the development of the seismic scale karst features through time, its impact on heterogeity in carbonate reservoirs and its contribution to the future carbonate exploration. References

Carr, D. L., Lancaster, D. E., Simmons, J. L., Elphick, R. Y., Pendleton, V. M. & Johns, R. A. (n.d). 3-D Seismic Evidence of the Effects of Carbonate Karst Collapse on Overlying Clastic Stratigraphy and Reservoir Compartmentalization. Retrieved on 15th March 2017 at http://dml.walden3d.com/twiki/pub/Correspondence/pdf/Boonsville/0000_Hardage_Boonsville_3D_Seismic_Effects_Carbonate_Karst_Collapse.pdfElkateb, T., Chalaturnyk, R. & Robertson, P. (2003). An overview of soil heterogeneity: quantification and implications on geotechnical field problems. Canadian Geotechnical Journal, 40, 1-15.

Garland, K., Neilson, J., Laubach, S. E. Whidden, K. J. (2012). Advances in carbonate exploration and reservoir analysis. Geological Society, 370(2012):115.

Kennedy, M. C. (2002). Solutions to some problems in the analysis of well logs in carbonate rocks. In: Lovell, M. & Parkinson, N. (eds.) Geological applications of wireline logs. American Association of Petroleum Geologists, 61-73.

Lucia, F. J. (2000). Origin and petrophysics of carbonate rock fabrics. 2000-2001 AAPG distinguished lecturers; abstracts AAPG Bulletin, 84,1879.

Montaron, B. (2010). Carbonate Evolution. Oil & Gas Middle East, 27-32.

Morse, J. W., Arvidson, R. S. & Luttge, A. (2007). Calcium carbonate formation and dissolution. Chemical. Reviews, 107, 342381.

Questiaux, J., Couples, G. D. & Ruby, N. (2010). Fractured Reservoirs with fracture corridors. Geophysical Prospecting, 58(2010):279-295.