Experimental Investigation of Lithology Variations on Shale Gas Generation and Retention From Immature Shales Maturation Under Anhydrous Gold-Tube Pyrolysis

Abstract

Source rock lithology variations to shale gas generation and retention remain unclear. In this study, immature Eagle Ford shale, Barnett shale, Woodford mudstone and Woodford chert were collected and represented four typical lithology variations. Small rock cylinders (diameter=5mm, length=20∼30mm) were drilled for all four types of shales, and a set of rock powder (grain size<0.074mm) from the Eagle Ford shale were prepared for comparison purpose. Both rock cylinder samples and rock powder samples were loaded into gold-tube reactors and isothermally heated for 72h at seven temperature points (130∼425°C) under a confining pressure of 68.0MPa, corresponding to a range of maturity levels: original state, bitumen formation, early oil, maximum oil, oil cracking, and early gas cracking stages. The principle difference between rock cylinder and rock powder in pyrolysis concerned the integrity of rock fabric and architecture of the given shale, and the difference in the measured gas yields between rock cylinder and rock powder were used to estimate the gas retention in source rocks. The results showed that gas retention within organic-rich shales was a function of thermal maturity. During bitumen formation stage and subsequent oil formation stage, C1-C5 gas yields from rock cylinder were significantly lower than those from rock powder while rock cylinder gave more CH4-rich gas in proportions. Whereas no obvious difference existed at oil cracking stage and early gas cracking stage. The decrease in the extent of gas retention with thermal maturity was attributed to the increased expulsion efficiency, which was driven by large quantity of oil and gas generation during maturation. The difference in lithology among the Eagle Ford shale, Barnet shale, Woodford mudstone and Woodford chert affected gas generation. Slightly higher C1-C5 gas yields from the Woodford mudstone at bitumen formation stage than those from the Eagle Ford shale and Woodford chert possibly indicated the influence of clay minerals. The presence of abundant clay minerals in the Woodford mudstone enhanced the gas generation appropriately by catalytic cracking of kerogen or heavy hydrocarbon compounds to short-chain hydrocarbons, resulting in more C2-C5 gas generation. However, the C1-C5 gas yields from the Barnett shale were considerably greater than those from the other three types in pyrolysis, mainly reflecting the difference of kerogen type other than the lithology variation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016