Impact of Dolomitization and Solid Bitumen on the Lower Triassic Shoal Reservoir, in Tieshan Gas Filed, Northeastern Sichuan Basin, Southwest China

Abstract

Abstract

The Lower Triassic Feixianguan Formation dolomitized oolitic shoal reservoir around the Liangping-Kaijiang paleo-embayment in the Sichuan basin is an enduring research target in China. In Tieshan gas field, the reservoir has been buried to a depth of up to 6500 m together with the other prolific gas-bearing traps, however, attracts few attentions as the porosity is a little lower than that of some other gas fields on the platform.

Based on thin sections and SEM characteristics, the main pore space is composed of intergranular porosity in the dolomitized shoal grainstone and packstone. In the zone of transition from partially dolomitized limestones to completely dolomitized dolostones at the top of the shallowing-upward subtidal carbonate sequence, the intergranular porosity and pore diameter increase together with the advancing degree of dolomitization. In the partial dolomitized part, the core of the oolite is composed of fine dolomite crystals, and the primary intergranular pore is hardly preserved among the line-contact oolites rimmed by deformed calcite cement. However, in the completely dolomitized part, both the concentric texture of oolite and the rim structure of calcite cement are obliterated, the intergranular pore occurs among the point-contact grains. By comparison, it indicates a distinct dissolution of rim cement.

The intact appearance of the oolite and rim cement in the limestone adjacent to pure dolostone precludes the pre-dolomite dissolution. The small amount of the post-dolomite calcite cement does not impact significantly on porosity, and the coexistence of quartz crystals and illite fiber indicates that the post-dolomite diagenesis system was a closed one in which significant burial dissolution did not happen. Dissolution during the process of dolomitization accounts for the present porosity.

Additionally, in the big-size pore of dolomitic reservoir intervals, 2 mircon thick film of solid bitumen on dolomite crystals prohibits the overdolomitization, hence benefits the reservoir porosity. Thus the solid bitumen film plays a beneficial role on the evolution of porosity.

This research suggests the dissolution during dolomitization contributes to the intergranular porosity system, and the oil cracking has a potential effect on the preservation of porosity. This model of reservoir development in Tieshan gas field provides a useful analogue to understand the evolution of shoal gas reservoirs that experienced dolomitization and oil cracking.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016