Source rock types, distribution and their hydrocarbon generative potential within the Paleogene Sokor-1 and LV formations in Termit Basin, Niger

The Paleogene lacustrine mudstone is one of the most important sets of source rocks in Termit Basin, Niger. However, studies on the Paleogene source kitchen are scarce. In this study, the source rock types, spatial distribution and their hydrocarbon generative potential within the Paleogene Sokor-1 and LV formations are systematically evaluated. A total of two third-order sequences (SSQ1 and SSQ2, from the bottom to top) and six systems tracts were identified in the Sokor-1 Formation, while the LV Formation mainly comprises pure shale and is regarded as a compositive stratigraphic sequence with no need of subdivision. Six types of source rocks could be distinguished within the three depositional environments of the sequence stratigraphic framework: (1) deep lake mudstones and shale deposited in the lacustrine deep-water facies, (2) shallow lake mudstones and carbonaceous mudstones occurring in the shallow lake environment, and (3) deltaic-front mudstones and prodeltaic mudstones developed in the deltaic facies. Deep lake mudstones/shale, which mainly occurred in the SSQ1-TST (transgressive systems tract), SSQ2-TST and LV Formation, are considered organic-rich source rocks with oil-prone generative potential. The shallow lake mudstones and deltaic mudstones were predominantly distributed in the tectonic slope and marginal areas of the Agadem Block within the lowstand systems tract and high stand systems tract of the SSQ1 and SSQ2. Nevertheless, geochemical results indicated that the deltaic mudstones are good gas-prone source rocks but the shallow lake mudstones were classified as poor potential source rocks. The variance in organic matter accumulation and preservation conditions within different depositional facies and the fluctuation of relative lake level are the controlling factors for the occurrence and distribution of high-quality source rocks. The effective Paleogene source kitchen is limitedly distributed around the depocenter of the Dinga Depression within a small scale, and therefore exploration for oil and gas resources from the Paleogene source kitchen should focus on the eastern Dinga fault-step zone adjacent to the depocenter of the Dinga Depression.

Structural Inversion, Reactivation and Extensional Detachment and Their Influence on the Formation and Preservation of Hydrocarbon Accumulations in Northern Western Desert of Egypt

ABSTRACT The northern Western Desert Basins of Egypt experienced a complex evolutionary history and several transformation of tectonic stress field properties. An integrated analysis of geological and geophysical data reveals that structural inversion, reactivation and extensional detachment develop in the area, and have a significant effect on formation and preservation of hydrocarbon accumulations. Such an analysis is paramount for prospect evaluation, risk mitigation, and therefore improving the exploration success rate. The rifting in Jurassic and early Cretaceous formed several faulted-depression basins with boundary normal faults in the area. A few boundary normal faults were reactivated in late Cretaceous to Eocene period as reverse faults with dextral compressive features, giving birth to a series of inverted anticlines over them. Compressive wrenching movement on the boundary faults greatly weakened their lateral sealing capacity and accordingly enhanced the vertical conduit capacity of hydrocarbon migration from the Jurassic source kitchen to Cretaceous inverted anticlines along the boundary faults. This is why Cretaceous inverted anticlines show a high concentration of hydrocarbon accumulations whereas there are few oil discoveries in the lower Alam El Bueib (AEB) formation and Jurassic along the boundary faults. Reactivation of basin boundary normal faults in late Tertiary to present abounds in the area. Most of them are surface penetrating, which are vital to the existing hydrocarbon accumulations because the reactivation could not only make poor the preservation of the existing hydrocarbon accumulations and cause the redistribution of hydrocarbons, but also it would destroy the existing hydrocarbon accumulations. Some unsuccessful wells can be attributed to the reactivation of basin boundary normal faults in late Tertiary to present. Some prospects associated with the reactivation of basin boundary normal faults have the same or similar hydrocarbon preservation risks as the unsuccessful wells. By integrating seismic interpretation and lithologic assemblage and thickness variation of the AEB formation, an extensional detachment fault was recognized in Alamein Basin. The detachment, located in the mudstone-dominated AEB 5-6 intervals, makes hydrocarbon migration difficult from Jurassic source kitchen to Cretaceous traps because the vertical migration pathways have been cut off by it, and are unfavorable for the formation of hydrocarbon accumulation above it.

Alkyl Dibenzothiophenes Tracing the Crude Oil Filling Pathway in the Fuyang Oil Layer of Wangfu Sag, Songliao Basin

Based on the thermodynamic stability and hydrogen bond mechanisms, alkyl dibenzothiophene molecular ratios, 4-/1-MDBT, and 4,6-/1,4-DMDBT, are used to characterize organic maturity and oil migration, which can act as molecular tracers for the filling orientation and pathway in oil reservoir. The results show that the well Shuang32 is the filling point which is close to the source kitchen. Three advantaged oil migration and filling pathways have been discovered, the first one is that oil is charged from well Shuang32 to well Shuang26- well Shuang301- well Shuang31 in northeast orientation, the second one is that oil is charged from well Shuang32 to well Shuang34- well Shuang36 in southwest orientation, the third one is that oil is charged from well Shuang 32 to well Chang502- well Shuang 23 in west-northwest orientation.

GEOCHEMICAL EVIDENCE OF MULTIPLE HYDROCARBON CHARGES AND LONG DISTANCE OIL MIGRATION IN THE VULCAN SUB-BASIN, TIMOR SEA

Hydrocarbon accumulation and migration processes in the Vulcan Sub-basin, Timor Sea, were investigated using the Total Scanning Fluorescence (TSF) technique and phenol and carbazole abundances in both reservoired and inclusion oils. The TSF spectral signatures have delineated multiple groups of oils within the Vulcan Sub-basin, largely identifying with the different oil families previously identified by biomarkers and isotope data. An apparent correlation between diminishing carbazole concentration and increasing distance from the source kitchen was found in reservoired oils within the Vulcan Sub-basin, providing evidence for long distance oil migration of up to 80 km. In the southernmost part of the Vulcan Sub-basin a possible light hydrocarbon charge rich in benzene and other low-molecular-weight aromatic hydrocarbons is believed to be responsible for the observed anomalously high phenol concentrations in oils from the surrounding wells. A previously unknown, possibly a Cretaceous-or-younger, source kitchen may be present along the eastern margin of the Vulcan Sub-basin and was probably responsible for the palaeo oil accumulations in the Tancred and Osprey structures as revealed by the TSF spectral signatures and biomarkers from the fluid inclusion oils.