Abstract:

In recent years, organic matter pores have been identified in shale and marlstone tight reservoirs, and act as an important reservoir space type in unconventional reservoirs. Evaluation of organic porosity is an urgent problem that needs to be solved in unconventional petroleum evaluation and exploration. This study takes marlstone in the Shulu Sag as an example, providing a method to evaluate organic porosity from the aspect of formation mechanism of organic pores. The marlstone in the Shulu Sag acts as both a source rock and a reservoir for petroleum and organic pores have been identified. Organic porosity is produced as the result of evolution of organic matter from solid immature kerogen to less dense fluid hydrocarbons during thermal maturation. The factors that control organic porosity include kerogen type, thermal maturity, initial total organic carbon, and source rock kinetics. The organic matter in the Shulu marlstone is predominantly Type Ⅰ and Type Ⅱ kerogen, and in a thermally mature zone. Test data show that the average present-day TOC is 1.66% and the average hydrogen index (HI) is 451 mg/g. The average temperature of maximum yield of pyrolysis (T_max) values is 444 ℃. Based on the test data, some other parameters were estimated. The average initial hydrogen index (HI_o) is 561 mg/g. The initial total organic carbon (TOC_o) is 1.9%. The convertible carbon accounts for 48% of TOC_o. The transformation ratio of convertible organic carbon is 31%. Using a mass-balance relation, the organic porosity was estimated, which is approximately 0.68%.

Key words: organic matter pore, unconventional reservoir, TOC_o, source rock kinetics, Shulu Sag