1997 Abrams Narimanov 1997

Marine and Petroleum Geology, Vol. 14, No. 4, pp. 451468, 1997 0 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain PII: s0294-9172(97)9991 l-l 0264-8172/97 17.00+0.00 Geochemical evaluation of hydrocarbons and their potential sources in the western. South Caspian depression, Republic of Azerbaijan Michael A. Abrams Exxon Ventures CIS), Inc., P.O. Box 4480, Houston, TX 77210-4480, USA Akif A. Narimanov SOCAR, Neftchilar Avenue 73, Baku 370004, Republic of Azerbaijan Received 5 September 1996; revised 6 December 1996; accepted 23 December 1996 The western South Cas pian depress ion, 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 elastic 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 elastic facies. Examination of oil molecular characteristics, oil-oil correlations, molecular characteristics of selected source rock samples, maturati on 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 lowto moderate organic maturities (VRE 0.75485) relati ve to convent ional peak generat ion windows. Significant variations in oil gravity and whole oil gas chromatogram character suggest post-emplacement bacterial and fractionation alteration. Sever al oils display characteristics consistent with multiple phases of trap charging. The bulk of reservoi red gases exami ned in this st udy have bee n 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 f rom the thermal destruction of liquid hydrocarbons. We postulate multi-stage hydrocarbo n emplace ment 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 subsidencefrom Quat ernarytect onicactivityproduced additional hydrocarbon sto replenish older, depleted traps and charge newly formed traps. This late tectonic activity also extensively redistributed hydrocarb on accumulations, degassed some that were breached by faults, and destroy ed other oil pools. Thermal disequilibrium from the rapid subsidence delayed hydrocarbon generation and increased the minimum depth required for the onset of liquid hydrocarb on generati on. 0 1997. Published by Elsevier Science Ltd. Keywords: Azerbaij an; geochemistry; hydrocarbon systems; South Caspian The South Caspian depression is a Tertiar y back-arc basin located in the offshore Republic of Azerbaijan. Azerbaijan is located in the Caucasus region, bordering Russ ia, Georgia, Armenia, Turkey, Iran, and the Caspian Sea (Figure la) with approximately 86 600 km2 of land area and a population of 7 million. It is one of the oldest know n oil producing regions in the world with a rich and colorful petroleum exploration and production history dating back to Alexander the Great. The soldiers of Alex- ander the Great utilized these rich oil resources during campaigns in the IV century BC from shallow hand dug wells. The first oil well was drilled in Azerbaijan by a Russ ian engineer, Sem yenov, in the Bibi-Eibat area in 1848, 11 years prior to Colonel Drake’s oil well in Pennsylvania (Narimanov and Palaz, 1995). By 1913, Azerbaijan produced 83% of the oil within the former USSR (Narimanov and Palaz, 19 95). Most recently there has been increased interest in the area with the signing of several multi-billion dollar production sharing agree- ments including the Megastructure (Chirag, Azeri, and deepwater Guneshli) (Figure Ib). The South Caspian depression contains significant accumulations of oil and gas with discovered reserves estimated to be 20 plus GOE B (billion oil equivalent barrels) and an additional 20 plus GOEB undiscovered reserves making the South Caspian depression a world class petroleum system. It is estimated there are up to 25 km of sediments in the basin center (Nadirov, 1990). The relati vely low geothermal gradients (around 1.5”C lOOm_‘) from rapid burial provide favorable thermal 451

1997 Abrams Narimanov 1997

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