Abstract:

The reservoir of the Dainan Formation, Jinhu Sag is a set of clastic reservoirs with medium-low porosity and permeability. The secondary pore is the most important and effective pore type in the study area, and the determination of its genesis and distribution is the key step for predicting favorable reservoirs. Based on core, thin section and cast thin section data, scanning electron microscope imaging, and physical property measurements, we analyzed the rock type, reservoir property, pore type and distribution of secondary pores, and discussed the genesis of secondary pores in association with the dissolution conditions in this area, which will provide scientific guidance for predicting favorable reservoir and for reservoir exploration. Our study showed that clastic reservoirs mainly deposited in delta, fan delta and show-shallow lakes. Controlled by sedimentary facies, the reservoir rocks are complex and include mainly unequal-grained sandstone and pebbly sandstone, fine sandstone and siltstone, showing varying lithologies in different areas. According to the triangular plot, the sandstone is mainly feldspathic debris quartz sandstone. The compositional maturity is medium to low and the textural maturity is medium. The porosity and permeability peak values were 12.0%-14.0% and 1-10 mD, respectively. There are some differences in reservoir properties associated with different sedimentary facies and regions. The reservoir properties of delta front are most favorable, with an average ~19.4% porosity and ~134.2 mD permeability, and the lithofacies is mainly fine sandstone distributing in the western and central areas. The fan delta near Well Bian-1 in the southeast of the Tongcheng fault zone cames in second. The fan sandstone is gravel bearing mix-grained sandstone with an average porosity of ~12.5% and permeability of ~6.2 mD. The reservoir property of shore-shallow lake is least favorable. It deposited mainly siltstone with an average porosity of ~8.3% and permeability of ~2.3 mD. There are two types of pores, primary and secondary pores, in the Dainan Formation. The dissolution pores included inter and intragranular dissolution pores and mold pores. The intergranular dissolution pores such as calcite, dolomite, iron calcite, etc., formed after dissolution of intergranular carbonate cements. The dissolution pores in grains mainly formed by selective dissolution of feldspar and carbonate debris. The mold pores left by the dissolution of dolomite grains could be seen. There are few primary pores in the Dainan Formation, mainly in the form of residual intergranular pores after quartz enlargement and developed in shallow buried wells, such as Wells Xinzhuang-1 and Guan-1-1. The pores evolved from primary to secondary pores in the Dainan Formation. On the whole, reservoir buried less than 1100 m is mainly in the early diagenetic stage A and mainly developed primary pores; reservoir buried between 1100-1500 m is in the early diagenetic stage B with mixed pore section; and reservoir buried below 1500 m entered the middle diagenetic stage A when secondary pores replaced primary pores. There are three secondary pore developing zones in the reservoir. The first one is between 1200-1600 m and distributed in the relatively shallow to moderately buried well area; the second one is between 1800-2800 m; and the third one is between 2900-3000 m. Among them, the absolute secondary porosity of the first and second secondary pore developing zones is relatively high, indicating strong dissolution. The secondary pore development in the Dainan Formation was related to the dissolutions of calcite and dolomite cements, feldspar and carbonate debris. Mature hydrocarbon from source rocks charging into the reservoir was the main controlling factor for the secondary pore development, as carbonate cements provided the material basis for the dissolution, and the dissolution of feldspar in turn contributed to the development of secondary pore. The formation of secondary pore was also related to the transformation of clay minerals, and fault activity further promoted the development of secondary pore.

Key words: pore type, secondary pore, pore evolution, controlling factor, Dainan Formation, Jinhu Sag