--> ABSTRACT: Assessment of Thermal Evolution Stages and Oil-Gas Migration of Carbonate Source Rocks of Early Tertiary in Eastern Sichuan, China, by Organic Inclusion Analysis, by Jixi Shi, Li Benchao, and Fu Jiamo; #91022 (1989)

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Assessment of Thermal Evolution Stages and Oil-Gas Migration of Carbonate Source Rocks of Early Tertiary in Eastern Sichuan, China, by Organic Inclusion Analysis

Jixi Shi, Li Benchao, Fu Jiamo

The Jialinjiang Formation of early Tertiary in Sichuan, China, is a series of limestone and dolomite sediments deposited in a platform shoal environment. The diagenetic sequence and organic inclusions trapped in minerals of 95 samples from 20 drillings have been studied. At the late diagenetic stage, pale yellow organic inclusions consisted of liquid hydrocarbons disseminated in pore-infiltrating dolomite, and the homogeneous temperature of contemporaneous saline liquid inclusions possessing a low gas-liquid ratio was 86°C. This indicates the evolution of the organic matter had gone over the oil generating threshold and oil formation had initiated.

In the limestone formed at the late diagenetic stage, more brown-yellow organic inclusions were scattered and/or developed along with fissures, comprising 60-70% liquid hydrocarbons and 30-40% gaseous hydrocarbons. Contemporaneous saline liquid inclusions with gas-liquid ratios of 5-10% had homogeneous temperatures of 90°-130°C. These findings show that the organic material had entered a high evolution stage and oil migration had taken place on a large scale.

At the late tectonism stage, a large quantity of brown-black organic inclusions, consisting mainly of gaseous hydrocarbons, existed in infiltrated secondary gypsum and calcite veins. The homogeneous temperatures of the contemporaneous saline liquid inclusions with gas-liquid ratios of 10-15% were 138°-170°C, concentrated at 150°C, implying an evolution stage of postmaturation and a predominance of gas migration at 150°C. The thermal gradient was confirmed to be 3.1°C/100 m by inclusion analysis. Liquid hydrocarbons were formed at a depth of 2,903 m, and the depth for oil migration in large quantities is 3,548 m, whereas gas migration is at 4,338 m. The industrial reserve in this area is of the pore-fissure type. The study results also reveal that the fissures n connection with oil and gas opened vertically along strike at a temperature of 160°C and depth of 5,250 m when the fissures were open. These fissures were formed during the Himalayan Period.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.