AAPG Middle East Region Geoscience Technology Workshop

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Spatial Correlation of Oil Seeps and Structures in the Zagros Fold Belt

Abstract

New statistical evaluations on the spatial correlation of oil seeps and different structural features in the Zagros Fold and Thrust Belt are presented. The investigated area represents a NW-SE trending data corridor, located largely in western Iran and partially in southeastern Iraq. The corridor is ~1000km long and ~300km wide. Data used for this analysis stem from our proprietary GIS database and is a combination of public domain data and manually mapped structural elements such as faults, folds and lineaments, which are based on digital terrain data and satellite images. The following data were used for the analysis: 406 anticlinal axes, 5233 minor faults and lineaments, 98 neotectonic faults, 221 oil seeps and 96 hydrocarbon field polygons (82 oil fields and 14 gas fields). Gas seeps were not taken into account here, as they are much more difficult to observe and generally only poorly documented. The data were analyzed by testing spatial correlation of oil seeps and different structural elements. This was achieved by superimposing and comparing different layers within GIS software. Additionally, simple statistical analysis was carried out, to test seep occurrences for preferred structural positions. This was achieved by observing, extracting and statistically plotting data from the GIS project. NW-SE (~130-140° az.) is the dominant orientation of structural elements here, representing the main structural grain of the Zagros Orogen. Neotectonic faults seem to largely follow this main trend, however, exhibit additional trends towards “steeper” strike azimuths (NNW-SSE, ~170° az.). Apparently, this steeper trend is associated with oil seeps in several places. Possibly this fault set is prone to leakage forming conductive pathways, due to its orientation in the present-day stress-field and resultant kinematic attitude, which likely puts it under shearing mode (?Riedel shear). Based on statistical analysis of this data 56% of all oil seeps observed are associated with faults. Of these fault-related seeps 45% are associated with neotectonic faults. Furthermore, 19% of all oil seeps observed here are located at or near anticlinal plunges (within ~1km), while 81% are not associated with plunges. Plunges are typically areas of intense deformation and secondary faulting, prone to creating leakage. Only 29% of all oil seeps are associated with anticlinal axes. The large percentage of non-structurally related seeps could be explained as follows: (a) either there are smaller faults not captured by the remote sensing data, or, (b) some of the seeps that are spatially not associated with faults, could be related to breached carrier beds (i.e. independent of structural elements); or a combination of (a) and (b). Finally, of all 96 oil and gas fields investigated here only 15% exhibit surface seeps. In summary, based on this data set, only a small fraction of oil field anticlines have surface seeps. About one half of all oil seeps is associated with faults, the other half isn’t, so there is no clear correlation of oil seeps being associated with faults. Neotectonic shearing on ~N-S trending faults seems to facilitate oil seepage to the surface, a process that can affect or even destroy prospects.