| Lower Triassic Feixianguan reservoirs mainly consist of platform oolitic beach limestone in Yuanba area in the northeastern part of Sichuan Basin. On the base of macroscopic observation of core and lithic thin sections from 6 wells, combining with the analysis of SEM and EP, five exposure indications can be recognized in wide-distributing Feixianguan oolitic limestone during diagenesis: 1) meteoric freshwater calcite cementation, 2) dissolution (filling freshwater calcite within
oolitic dissolution pores), 3) gravelly oolitic limestone (gravels are the broken beach rock), 4) vadose clay and silt and 5) ferritization of ooids. Combined with the characteristics of regional geological and tectonic evolution, it is shown that sea level decline during the end of Feixianguan 2 Member sedimentation had made the primary sedimentary region expose to surface and undergo meteoric freshwater leaching and dissolution. Contributing to exposure and dissolution, a great amount of secondary pores develop increase which plays an important role in constructing excellent reservoirs. These exposure indications can be also used to recognize the sequence interfaces in carbonate rock and confirm the division of fine sequences. |
