Evolution of Tight Gas Sandstone Reservoir Properties and Driving Force in Eastern Kuqa Depression, Tarim Basin

Abstract

The Kuqa Depression in the Tarim Basin is a prolific tight sandstone gas-producing area in China. In the east of the depression, the assessed tight sandstone gas resource is about 1×1012 m3. In order to more accurately analyze the accumulation process of tight sandstone gas reservoir, the reservoir properties and pressure are reconstructed as the starting point, and based on an integrated investigation of the characteristics and porosity evolution of sandstone reservoir, hydrocarbon charge history, and overpressure evolution, the relationship between hydrocarbon charge and tight sandstone reservoir formation is analyzed. Moreover, the contribution of source-reservoir pressure difference is quantitatively evaluated as a driving force in the process of gas accumulation. Collective evidence indicates that the Lower Jurassic Ahe Formation tight sandstone reservoir in the eastern Kuqa Depression is strongly heterogeneous: the reservoir space is dominantly intergranular and intragranular dissolution pores; the development of fractures could improve local reservoir properties, while compaction mainly led to the formation of tight reservoir. Evolution of reservoir properties shows that the lower Jurassic stratum was compacted at circa 8Ma before the Kuqa age. Fluid inclusion analysis shows that gas charge reached its peak during 5~2Ma. Therefore, reservoir compaction (8Ma) occurred before the peak of gas generation (5Ma). Comparison of the source-reservoir pressure difference (driving force of reservoir formation) derived numerically and the critical charging pressure (hydrocarbon accumulation resistance) obtained in physical experiments shows that overpressure increased dramatically and caused generally larger source-reservoir pressure difference than the critical gas charging pressure at 5.2Ma, hence resulting in large-scale hydrocarbon generation. The time is also consistent with the results of fluid inclusion analysis. At circa 1.6Ma, hydrocarbon-generating pressurization was weakened and overpressure was released by the faults, leading to the decrease of source-reservoir pressure difference. The concept of “filling-amount” is proposed to quantitatively characterize the contribution of source-reservoir pressure difference in the process of hydrocarbon accumulation. The result of data analysis indicates that the “filling-amount” is generally higher in the Dibei area, which is the key area of exploration for tight sandstone gas in the eastern Kuqa Depression.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017