Surface mapping of the Milh Kharwah salt diapir to better understand the subsurface petroleum system in the Sab’atayn Basin, onshore Yemen

Abstract

In 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 targeting the fractured basement play in the Sab’atayn Basin. Based on numerous well penetrations, the presence of non-evaporitic lithologic units such as black shales, marls, carbonates, and sandstones within the Tithonian Sab’atayn Formation is quite common and quite important for the prolific Alif oil play. The internal lithologic and structural complexity of the Tithonian evaporites was addressed by analyzing a few outcropping salt diapirs east of the Habban Field area in the central part of the Sab’atayn Basin. In particular, the Milh Kharwah diapir was studied by recently acquired high-resolution satellite images (16 band VIS-NVIR and SWIR WorldView-3 imagery) integrated by outcrop sampling of the various lithologies exposed in the diapir. The Sab’atayn Formation was found here to be a “dirty salt” as it has not only halite, anhydrite, and gypsum but also various stringers of bituminous marls and other non-evaporitic lithologies. Based on the lithologic composition of 10 surface spot samples and their spectral signal in the infrared satellite data sets, a quasi-geologic map of the diapir was generated. The remote sensing evaluation was done in lieu of the time-consuming and logistically challenging surface geologic mapping in this conflict-stricken region. The geometry of the intra-salt curtain folds seen on the new map, considered as a useful approximation of a geological map, is very characteristic for stems of diapirs seen elsewhere, suggesting significant post-kinematic exhumation and erosion in the area. Salt diapirs reaching the surface dissipate heat more efficiently and thus keep deeper segments of a basin relatively colder and potentially within the oil window for a longer time. Recent apatite fission-track analysis using drill-core samples from four nearby wells provided middle to late Eocene ages (34–39 Ma) for the onset of an estimated 400–500-m erosion of the Milh Kharwah salt diapir constraining our petroleum system model. Moreover, the quantitative lithologic description of the evaporite sequence, integrating the surface samples with high-resolution satellite images, further refined our basin modeling results.