--> --> Surface observations of salt tectonics to better understand the subsurface petroleum system in the Sab’atayn Basin, onshore Yemen

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Surface observations of salt tectonics to better understand the subsurface petroleum system in the Sab’atayn Basin, onshore Yemen

Abstract

In the central part of the Sab’atayn Basin of Yemen hydrocarbons were generated from pre-salt Upper Jurassic source rocks during the Cenozoic and the salt provides the ultimate super seal for the pre-salt and intra-salt traps. Therefore the proper understanding of salt tectonics is critical for ongoing hydrocarbon exploration efforts in the Sab’atayn Basin. Based on the large number of well penetrations, the presence of non-evaporitic lithologic units such as neritic carbonate, coarser siliciclastic and shaly intercalations within the Sab’atayn Formation is the function of the depositional environment within the rift basin. Anhydrite and coarse clastics are common along the basin margin whereas massive halite and neritic carbonates are more frequently drilled in the basin center. Non-evaporitic lithologies within the Sab’atayn Formation are quite common and are quite important for the prolific Alif play. The internal lithologic and structural complexity within the Sab’atayn Formation was studied by analyzing a few outcropping salt diapirs east of the Habban Field area. The Milh Kharwah diapir was studied by high-resolution satellite images integrated by field sampling of the various lithologies within the salt diapir. The Sab’atayn Formation was found here to be a "dirty salt" in the sense that it has not only halite, anhydrite and gypsum, but various stringers of bituminous marls and other non-evaporitic lithologies, all arranged in drape and curtain folds within the diapir. Based on the lithologic composition of the surface spot samples and their spectral signal in the high-resolution infra-red satellite data sets, a very detailed geologic map of the diapir was compiled. This evaluation was done in lieau of the time-consuming and logistically challenging surface geologic mapping. The geometry of intra-salt folds seen on the new geological map, with subvertical fold axes, is very characteristic for the stems of diapirs suggesting significant post-kinematic erosion in the area. Since evolving salt diapirs can modify significantly the thermal regime around them, the thermal anomalies then can modify the maturation of source rocks nearby. In particular, salt diapirs reaching the surface dissipate heat more efficiently and thus keep deeper regions of the basin relatively colder and potentially within the oil window for a longer time. The cooling effect is maximized when the top of the salt diapir remains at or very close to the surface for a significant period of time. In the case of the Milh Kharwah salt diapir the Upper Jurassic salt could have reached the surface as early as the latest Cretaceous. Our basin modeling study shows that besides the various timing scenarios, the lithologic composition of the "salt" could make a big difference as to the hydrocarbon phase in the present-day pre-salt trap, i.e. whether it is a light oil, condensate or gas.