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AAPG Bulletin

Abstract

AAPG Bulletin, V. 105, No. 11 (November 2021), P. 2181-2205.

Copyright ©2021. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/06032119263

Understanding lateral and vertical fluid variations in the Pliocene sandstone reservoirs in the eastern South Caspian Basin

Sabine Mehay,1 Mohamed Hashem,2 Lamia Rouis,3 Eldar Mollianiyazov,4 Barry Bennett,5 and Artur Stankiewicz6

1Asset Consulting Services, Schlumberger, Aachen, Germany; Reservoir Sampling and Analysis, Schlumberger, Jebel Ali Free Zone, Dubai, United Arab Emirates; [email protected], [email protected]
2Dragon Oil, Emirates National Oil Company (ENOC) House II, Dubai, United Arab Emirates; present address: X.Shell B.V., Wassenaar, Netherlands; [email protected]
3Dragon Oil, ENOC House II, Dubai, United Arab Emirates; [email protected]
4Dragon Oil, ENOC House II, Dubai, United Arab Emirates; [email protected]
5Reservoir Sampling and Analysis, Schlumberger, Calgary, Alberta, Canada; [email protected]
6Reservoir Sampling and Analysis, Schlumberger, East Ahmadi, Kuwait; present address: Zgorzelec, Poland; [email protected]

ABSTRACT

The South Caspian Basin hosts more than 600 onshore and offshore oil and gas fields characterized by large variations in fluid properties impacting field development and nearby exploration efforts. A total of 32 surface and downhole fluid samples collected from six distinct Pliocene sandstones in an offshore anticline field in the eastern South Caspian Basin were analyzed (gas, light- and midrange–fraction composition, biomarkers, and bulk and gas compound-specific carbon isotopes) to investigate the processes controlling the fluid properties at the field scale. Sampling and sample handling methodologies enabled preservation and recovery of representative light-end compositions.

 Lateral fluid property variations within the main productive sandstone reservoir (API gravity from 36.8° to 53.4° and gas–oil ratio from 544 to 947 SCF/bbl) were found to result from the combined effects of biodegradation, water washing, and a light charge contribution, the latter having the predominant impact on the observed fluid properties variations. Ongoing migration of higher-maturity fluids generated deeper in the South Caspian Basin or evaporative fractionation, which likely affected fluids in the deeper sandstones, may be responsible for the contribution of a volatile enriched charge to the shallow reservoirs. A comparison based on source and maturity diagnostic parameters with Pliocene reservoir oils from the western South Caspian Basin is consistent with a similar reservoir filling history but more complex migration pathways for the eastern South Caspian Basin reservoirs.

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