Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China

摘要

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin. Their occurrence, like other cases worldwide, can be divided into two locations in general, i.e., platform interior and platform margin locations. Their differences of reservoir features and origins, however, have not been investigated comprehensively due to different exploration degrees. This issue is addressed in this paper, to provide basic data and information for the basin’s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide. We compared the features of these two types of reservoirs in detail, including the depositional and diagenetic features, pore types and petrophysical features. Based on the comparison, the origin of the reservoirs was further discussed. It is shown that the reservoirs in platform interior and platform margin locations differ significantly. The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone, which was weakly compacted and intensely cemented and has undergone meteoric dissolution. Pore types include intragranular dissolution and moldic pores, with low porosities (<6%) and low permeabilities ( 9% porosity and >0.1 mD permeability) and pore-throat structures than the interior reservoirs. These differences were influenced by both primary depositional features and secondary diagenesis. For the interior carbonate reservoirs, early meteoric dissolution, weak compaction and strong cementation are important controlling factors. By contrast, the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization, preservation of primary pores and burial dissolution.