Integrated sequence stratigraphy and geochemistry of the Triassic Shublik Formation in the Alaska North Slope: implications for petroleum expulsion potential

Abstract

The recent discovery of petroleum in the Cretaceous strata has gained an interest to study the source of the petroleum fluids mainly generated by the Triassic Shublik Formation in the Alaska North Slope. The Shublik Formation is known as one of the key source rocks in the Alaska North Slope which is one of the most prolific oil and gas producing basins in North America. The study area covers 18,993 km2 from central to western part of the Alaska North Slope and utilizes well logs, total organic carbon (TOC), Rock-Eval pyrolysis, vitrinite reflectance, biomarker, and carbon isotopes data. The Shublik Formation present across the study area can be divided into three large stratigraphic sequences that include the lower clastic (LC) unit, the middle carbonate-chert (MCC) unit, and the upper clastic-carbonate (UCC) unit comprising five transgressive-regressive depositional sequences. The isopach map illustrates that the Shublik Formation has an elongated south-southwest trend and thickness varies in the study area due to a complex paleogeography setting. The UCC unit has the greatest thickness and the LC unit is very thin (<50 ft). Total organic carbon content (TOC) has been calculated from well logs and calibrated with available measured geochemical data. Out of the three stratigraphic sequences, the MCC unit is the most organic-rich interval followed by the UCC unit and the LC unit. The present-day thermal maturity exceeds mature-postmature window towards south of the study area (also supported by decreasing Hydrogen Index (HI) as the maturity increased). Biomarkers and carbon isotopes data indicate that the Shublik Formation is dominantly marine with mixture of organofacies A (clay-poor) and B (clay-rich) and more of organofacies B present in the eastern part of the study area. The mass balance restoration has been done in order to produce the maps of original (i.e., prior to thermal maturation) TOC and HI distributions. In general, there is better quality of organic matter in the Shublik Formation in more distal setting towards south of the study area. However, the northeastern part of the study area shows that organic-rich rocks are also present in more proximal setting. This might have been caused by the variability of source rock preservation and redox conditions in the Shublik Formation. The ultimate expellable potential (UEP) maps were constructed for the Shublik Formation and stratigraphic sequences in the study area. The UEP can be divided into the ultimate expellable potential of oil (UEO) and the ultimate expellable potential of gas (UEG). The UEP of the Shublik Formation ranges from 8-12 mmboe/km2 (4-8 mmstb/km2 UEO and 2-6 mmboe/km2 UEG) with thickness 61-91 m (200-300 ft) in the area of Pikka-Horseshoe discovery (Cretaceous reservoirs). The contribution of stratigraphic sequences indicates that the MCC unit has the major contribution in expelled petroleum followed by the UCC unit and the LC unit. In the northwest of the study area, the source rocks tend to have lower UEP (<2 mmboe/km2) and produce relatively more gas. The thickest interval in the basin depocenter of the Shublik Formation also illustrates that thicker interval does not always result in best source rock potential because thickening may be accompanied by clay mineral dilution leading to lower UEP. The petroleum generation and expulsion in the study area are controlled by the source rock organofacies, quality, and thickness. The petroleum was generated during the Cretaceous time, and the peak of the oil window occurred in the Early Cretaceous (approximately 76 ma) based on the burial history model from the Inigok #1 well in the southern part of the study area.