This study examines an important channel sand reservoir in the Junggar Basin, which has a reservoir quality that varies greatly in the vertical and horizontal directions due to the presence of tight sandstones deposited under different sedimentary conditions. Studies confirmed that the high-energy sandstone deposits have good porosity and permeability based on the core data. The channel sand is 20 m to 30 m thick in the Qigu formation and can be divided into three to four channels. Identifying and characterizing the high-energy sandstone "sweet spots" was the objective of this case study.
In order to find the sweet spot in this reservoir, high-definition oil-base mud microresistivity image logs were acquired in five exploration wells. The image logs made it possible to identify the different kinds of sandstone reservoirs based on sedimentary structure and texture. Parallel bedding, deformed bedding, cross bedding, and massive bedding were all identified. The paleocurrent energy was from high-to-low and from bottom-to-top in a single channel sand body. The core and production data validated the reservoir qualities, which were identified from the sedimentary features.
The channel sand was divided into three to four multistory sand bodies based on image logs. As a result, the typical channel-levee system—composed of massive bedding, cross-bedding and parallel bedding features—was revealed. The massive bedding was mainly at the bottom of the single channel system, surmounted by high-angle cross-bedding, and then finally the parallel bedding intervals. The paleocurrent energy changes from high-to-low vertically within the single channel system. Based on the open-hole and image logs, two types of sweet spots were identified: massive bedding sandstone and high-angle cross-bedding sandstone. These two types were deposited in a high-energy environment which formed the potential sweet spot in the channel complex. The massive bedding intervals, which are mainly in the channel bottom, when cemented by calcite or clay, will become tight sometimes. The high-angle cross-bedding sandstone and uncemented massive-bedding sandstone were the potential target zones for the horizontal well.
This case study will show the interpretation of sedimentary features in the channel sand reservoir based on high-definition image logs, to provide a comprehensive understanding of the geologic features that differentiate reservoir quality. The identification of cross-bedding in the channel sand provided a new way to predict possible tight sandstone sweet spots that can help to design horizontal wells.