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Abstract: Petroleum Geochemistry of Atrau Region, Pre-Caspian Basin, Kazakhstan

Kadir Gurgey

Pre-Caspian basin covers an area of approx. 500,000 sq km, and is characterized mainly by thick (0-5000 m) Kungurian salts. Atrau region occupies 100,000 sq km, and is located at the southern part of the basin. Oils of this basin are found in the sub-salt (Carboniferous reefs) and supra-salts (Triassic red beds and Jurassic- Cretaceous clastics) reservoirs. Seventeen crude oil samples analyzed from different wells appear to be paraffinic and paraffinic- naphthenic type. Some of the oils hardly contained any n-alkanes, probably owing to biodegradation. Biomarker signatures of saturate and aromatic fractions and stable carbon isotopes of whole oils revealed two genetically different oil families; family I and family II. Family I was generated from elastic supra-salt sedimen s having immature (%Rc=0.55) terrestrial organic matter. Family II was generated from carbonate rich sub-salt sediments, containing mature (% Rc=0.65-0.80) marine organic matter.

Majority of Triassic, Kungurian and Upper Cretaceous successions contained enough organic matter with considerably low total petroleum potential (S1+S2). Upper Carboniferous sediments, on the other hand, contain enough and oil prone organic matter that reached peak oil generation stage (233.1 Ma) and hydrocarbon saturation level for expulsion as a result of high sedimentation rates in the Lower to Middle Triassic succession in Kobyekovskaya-2 well. Maximum paleotemperature reached in the area was not enough for H2S formation and cracking of already generated hydrocarbons to natural gas.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France