Geochemical evaluation of hydrocarbons and their potential sources in the western South Caspian depression, Republic of Azerbaijan

Abstract

The western South Caspian depression, located in offshore Azerbaijan, contains significant accumulations of oil and gas in Upper Tertiary fluvial-deltaic sediments. The active Tertiary hydrocarbon system is a product of unique paleogeographic and tectonic events that led to Paleogene deposition of organic-rich source rocks, rapid Pliocene subsidence concurrent with voluminous supply of clastic sediments, and development of anticlinal traps with abundant shale diapirs. Molecular characterization of selected oil samples indicates most of the oils are sourced from similar organic facies. The molecular characteristics are consistent with oils sourced from a Tertiary, slightly calcareous, marine clastic facies. Examination of oil molecular characteristics, oil-oil correlations, molecular characteristics of selected source rock samples, maturation models, and potential migration pathways suggests the oil is not syngenetic, but most likely sourced from deeper Miocene and older marine shales. Most of the oils have low to moderate organic maturities (VRE 0.75–0.85) relative to conventional peak generation windows. Significant variations in oil gravity and whole oil gas chromatogram character suggest post-emplacement bacterial and fractionation alteration. Several oils display characteristics consistent with multiple phases of trap charging. The bulk of reservoired gases examined in this study have been sourced from mixed terrestrial-marine kerogen generated at organic maturities of VRE 0.80–1.00 with some biogenic-low maturity mixing. None of the gases examined were sourced from the thermal destruction of liquid hydrocarbons. We postulate multi-stage hydrocarbon emplacement into evolving structural traps. The first phase of emplacement occurred in the Middle Pliocene when tectonic movement and increased subsidence initiated early trap/reservoir formation, hydrocarbon generation, and migration. Late rapid subsidence from Quaternary tectonic activity produced additional hydrocarbons to replenish older, depleted traps and charge newly formed traps. This late tectonic activity also extensively redistributed hydrocarbon accumulations, degassed some that were breached by faults, and destroyed other oil pools. Thermal disequilibrium from the rapid subsidence delayed hydrocarbon generation and increased the minimum depth required for the onset of liquid hydrocarbon generation.