Application of geothermal gradient in the study of thermal evolution of Paleozoic source rocks, Tarim Basin

Abstract

Considerable debate has arisen regarding studies of the thermal history of Paleozoic source rocks in the Tarim Basin, particularly regarding the thermal evolution of ultra-deep, ancient source rocks. The evaluation method for the thermal evolution stages has long been a key issue in the study of source rocks' thermal history. Research findings revealed that the Cambrian paleo-geothermal gradient in the Tarim Basin varies between 2.95 and 3.6 ℃/hm. Based on the hydrocarbon generation threshold temperature of 65 ℃, the required overlying strata thickness ranges from 2 203 to 1 806 m, with a difference of 397 m, equivalent to 2.58 ℃, showing a relatively small temperature difference. Thus, it is considered that the variation in the Cambrian paleo-geothermal gradient in the Tarim Basin is minimal, allowing the differences in geothermal gradient values for each period to be ignored, and that thickness may be the main determinant of temperature. The sedimentation and residual thickness of the strata can be accurately obtained, with the error mainly depending on the restoration of the denudation thickness. The thermal evolution stages of the Cambrian Yurtus Formation source rocks in the Tarim Basin were evaluated using reliably obtained parameters such as strata thickness, paleo-geothermal gradient, denudation thickness, and hydrocarbon generation charts of source rocks. With this assessment, the effective source rocks and their distribution in the key period of the Manjiaer Depression were delineated. Moreover, the effective source rocks ofthe Yurtus Formation, their large-scale hydrocarbon generation capacity, and the favorable areas for late-stage hydrocarbon accumulation were predicted and classified. The effective source rocks at different stages and their large-scale hydrocarbon generation potential are crucial for hydrocarbon accumulation in each period. This method avoids the issue of unreliable temperature scales in maturity identification, providing an important scientific basis for deep and ultra-deep oil exploration and development in the Tarim Basin and for selecting favorable areas for late-stage hydrocarbon accumulation.

Keywords

• geothermal gradient
• maturity
• favorable area prediction
• effective source rock
• yurtus formation
• cambrian
• tarim basin