Abstract: Petroleum Systems of the Southwest Caspian Basin
Michael A. Abrams, Akif A. Narimanov
The Southwest Caspian basin, located in offshore Azerbaijan, contains significant accumulations of oil and gas in Upper Tertiary siliciclastic sediments. The central basin contains up to 25 km of sediments. The relatively low geothermal gradients and low degree of compaction from rapid burial provide favorable conditions for the retention of hydrocarbons at relatively great depths. A variety of structural styles occur, ranging from anticlinal folds to monoclines, with various degrees of reverse faulting and brecciation. These structural features occur along clearly identifiable trends associated with underlying deep seated faults. Many structures are penetrated by mud diapirs and mud volcanoes. The majority of the accumulations are located in deltaic Middle Pliocene sedim nts (Productive series).
Molecular characterization of selected oil samples indicate most of the oils have been sourced from the same or similar facies; a Tertiary Type II, slightly calcareous, marine clastic facies. Insufficient organic-rich rocks are available for a reliable oil-source correlation. Examination of oil molecular characteristics, oil-oil correlations, molecular characteristics of key stratigraphic horizons, paleofacies maps, maturation, and potential migration pathways suggest the oil was not syngenetic but most likely sourced from deeper Oligo-Miocene or older marine shales. Most of the oils have been generated at low to moderate maturity levels (VRE 0.75 to 0.85). The oil specific gravities range from relatively low to high. This large variation in part is related to post-generative secondar alteration. Many of the lower gravity oils have experienced evaporative fractionation from pressure reduction due to erosion and/or faulting or have been altered by relict bacterial degradation. In addition, there are several groups of oils which display characteristics consistent with double charging. Compositional data for a single offshore gas sample suggest the gas is a mixture of low maturity Type III and biogenic. The gas is almost entirely hydrocarbon gas with only a small portion of non-hydrocarbon gas.
A multi-stage model of hydrocarbon emplacement for evolving structural traps has been postulated. The first phase of emplacement occurred in the Middle Pliocene when tectonic movement and significant subsidence initiated early trap/reservoir formation, migration, and hydrocarbon generation. Late Quaternary tectonic activity led to the replenishment of older depleted traps, additional hydrocarbons for enhanced traps, and charging of new traps. In addition, late tectonic activity caused extensive redistribution of hydrocarbon accumulations, degassing due to breached faults, and destruction of selected oil pools.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France