AN ATTEMPT TO RECOGNISE OIL POPULATIONS AND POTENTIAL SOURCE ROCK TYPES IN PALEOZOIC SUB- AND MESOZOIC-CENOZOIC SUPRA-SALT STRATA IN THE SOUTHERN MARGIN OF THE PRE-CASPIAN BASIN, KAZAKHSTAN REPUBLIC

Abstract

Fifty-six rock and 28 crude oil samples from the sub-salt and supra-salt section of the southern Pre-Caspian Basin were analyzed by organic geochemical means. Eighty-five percent of the rock samples from the Cretaceous to Carboniferous section contain predominantly gas-prone mixed Type II/III organic matter. Rock-Eval Tmax values of selected samples range from 437 °C for Triassic and 449 °C for Kungurian samples. The oil sample set represents two geographically distinct oil populations with one containing two sub-populations termed Population 1A and Population 1B. Fourteen low sulfur-bearing, moderately mature oils grouped into Population 1A are found to the east and west of the town of Atrau. Excluding oil samples KE and O they represent lightly to moderately biodegraded supra-salt and non-biodegraded sub-salt oils. Oil to source correlation indicates an origin from Artinskian marine siliciclastic source rocks containing mixed type II/III organic matter deposited under dysoxic conditions. High sulfur-bearing, non-degraded Population 1B consisted of 13 oils from supra-salt reservoirs occurring in an area northeast of Aktau. These oils most likely originated from an organic matter type similar to that which sourced the Population 1A oils. However, the higher sulfur content, supported by several lithology-sensitive biomarkers present in the Population 1B oils, suggest a carbonate rich source rock. It is proposed that Population 1B oils may have originated from age-equivalent marine carbonate to clay-rich Artinskian clastic sources. Population 2 oil from the northwestern Atrau area is found in a supra-salt reservoir. It is thought to originate from a Triassic supra-salt non-marine clastic source rock containing mixed type II/III organic matter deposited under dysoxic and less saline conditions.