LITHOFACIES, DIAGENESIS AND RESERVOIR QUALITY EVOLUTION OF WOLFCAMP SANDSTONE-SILTSTONE SUCCESSIONS IN DELAWARE BASIN, WEST TEXAS

The Lower Permian Wolfcamp Formation in the Delaware Basin is an important hydrocarbon reservoir. Based on cores, thin sections, and SEM observations, this study reports the lithologic and diagenetic characteristics of reservoir. Diagenesis of Wolfcamp reservoirs comprises compaction, cementation and grain dissolution. Among the diagenetic minerals, carbonate and quartz forms the major cements clean sandstones, whereas the mixed-layer illite/smectite and illites are the dominant cements in clay-rich sandstones. The primary porosity is rare, and the secondary pores are of great importance for the Wolfcamp reservoirs. The micro-pores are mainly associated with authigenic clay minerals.

Five types of lithofacies are defined by microscopic observations. They are clean dissolved sandstones (Lithofacies A), quartz-cemented sandstones (Lithofacies B), clay matrix-rich sandstones or siltstones (Lithofacies C), carbonate-cemented sandstone (Lithofacies D) and illite-smectite-cemented sandstones (Lithofacies E). Then the typical diagenetic modifications and its evolution sequence for the five defined lithofacies were constructed based on textural relationships. Mechanical compaction is the main mechanisms for porosity reduction in early diagenesis, and the reduction rate is highest in the clay matrix-rich sandstones or siltstones. In contrast, carbonate cements that formed prior to compaction in Lithofacies D produce a sufficient framework grain-supported rock for overburden mechanical compaction. Progressive burial and mechanical compaction resulted in the rapid decrease in porosity in Lithofacies C. Some dissolution occurred during early diagenesis in Lithofacie A, and the majority of feldspar reactions occurred during burial diagenesis associated with organic acids. The transformation of smectite to illite through intermediate phases of mixed-layer illite/smectite in Lithofacies E is the typical mesogenetic modifications. The quartz cements are mostly abundant in Lithofacies B. The burial diagenesis in Lithofacies D is dominated by late-stage carbonate cements filling into the small pores. The reservoir-quality evolution pathways of various lithofacies are predicted considering the variations of grain size, sorting, shape, and matrix content within various Lithofacies.