Abstract: Geochemical Characteristics of Lacustrine Carbonate Source Rocks
JINCAI, TUO
Lacustrine carbonates in Biyang Depression, China, develop mainly in the middle and lower parts of Hetaoyuan Formation, Tertiary, with a maximally accumulated thickness of 2211 m. Oil produced in Yun-1 and Yun-2 wells is proved to be sourced from this sequence of carbonates. The main lithologies of the carbonate beds are dolostone, muddy dolostone, siltstone and calcareous mudstone, their maximal and minimal acid-solubles is 87.8% and 33.3%, respectively. The total-organic-carbon content of possible lacustrine carbonate source rocks average 1.75% and range from 0. 34% to 4.87% and extractable-organic-matter content range from 0.0287% to 1. 0408%. Elemental analysis of kerogen shows that the most atomic O/C rations in possible lacustrine carbonate source rocks are lower than 0.05, while the atomic ratios of H/C range from 0.63 to 1.2. The position of the samples under studied on the modified van krevelen diagram corresponds to oil-prone organic matters. Organic petrography show the organic matters are mainly composed of lower organism such as bacteria and algae and parts of remains of higher plants strongly reworked by bacteria. Maturity research suggests that the organic matters from all possible lacustrine carbonate source rocks are not of high maturity, the largest vitrinite reflectance value is 0.62% and sterane (alpha)(alpha)C[29]20S/20(S+R) ratios are less than 0.46.
The compositional characteristics of extractable-organic-matters of the possible lacustrine carbonate source rocks are rich in asphaltene and resin (greater than 50%) and are short of saturated and aromatic hydrocarbons (less than 50%). The analysis of biomarkers of extractable-organic- matters shows a Ph/Pr domination, high content of gammacerane, low contents of pregnanes and rearranged steranes, and a nearly equal content of C[27] and C[29] steranes.
AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid