Formation of oil and gas fields in Tertiary lacustrine basins in Thailand

Abstract

Tertiary basins in Thailand are pull-apart basins which originated in coincidence with collision of Indian Plate and Eurasian Plate at the end of Cretaceous age. Lacustrine sediments of Oligocene age are commonly recognized in the lower part of the basins, which are known to act as source rock for crude oil both in onshore and offshore areas. Based on geochemical investigation of the basins, two characteristic patterns of petroleum system from the lacustrine source rock were recognized as (1) syn-rift type and (2) post-rift type. In the syn-rift type, source and reservoir rocks are interfingering, or reservoirs are located immediately above the source rocks, which lead to lateral migration or short vertical migration of hydrocarbons. In the post-rift type, source rocks are present at greater depth than reservoirs and, therefore, hydrocarbons should migrate vertically to shallower reservoirs and oil reservoirs and gas reservoirs coexist. Tertiary basins in Thailand originated with common lacustrine source rocks at the bottom. However, differences in the subsequent tectonic movement lead to the variations in depth and maturation level of the source rocks and reservoir type. Consequently two different types of petroleum system were formed.