Abstract
The Lower Jurassic Ahe Formation is an important exploration target for deep clastic reservoirs in the eastern Kuqa Depression. The Ahe Formation sandstones show heterogeneous porosity and permeability petrophysical properties. These properties have been poorly understood, limiting forecast of petroleum accumulations and making it difficult to develop the reservoirs. Based on cores, thin sections, SEM, and fluid inclusions, this study examined sandstone composition and texture and diagenetic heterogeneity at the core scale. The aim was to understand the influence of variations in detrital composition and texture on diagenetic and reservoir quality evolution. The Ahe Formation sandstones are dominantly fine- to coarse-grained litharenites, with minor feldspathic litharenites. In fining-up sand beds, detrital grain size determines the degree of mechanical compaction and, consequently, the abundance of porosity through ductile grains and muddy matrix. Local complete calcite cementation is a noticeable exception to this general trend. Three sandstone petrofacies have been defined based on texture and framework composition, detrital matrix, diagenesis, and pore types: (1) ductile-lean sandstone, (2) ductile-rich sandstone, and (3) tightly calcite-cemented sandstone. Different petrofacies experienced contrasting diagenetic and porosity evolution pathways. Ductile-lean sandstones underwent lower degree of compaction relative to ductile-rich sandstones during the eodiagenesis stage, and extensive grain dissolution occurred. The petrofacies remained relatively porous and permeable before early oil arrival. With continued burial, the porosity and permeability in the sandstones were further reduced by cementation. The petrofacies still had moderate porosity and permeability and were substantially charged when late petroleum migrated into the reservoirs. Thus, ductile-lean sandstones constitute effective reservoir rocks in deep reservoirs. By translating petrofacies to signatures of well logs, the effective properties of the reservoir rocks can be forecasted at the well scale.
Highlights
The exploration and development of deep clastic reservoirs have obtained more attention in China
K-feldspar grains are less than 18% with an average of 8%, and plagioclase grains are less than 10% with an average of 1%
Petrographic observations indicate the following: those microfractures along grain boundaries accounting for 40%, those cutting through rigid quartz and feldspar grains account for 39.2%, those cutting through ductile rock fragments make up 9.6%, those cutting through clay minerals make up 5.6%, those cutting through other cements make up 0.8%, and those cutting through pores account for 4.8%
Summary
The exploration and development of deep clastic reservoirs have obtained more attention in China. Most deep sandstones experienced multistage tectonic/burial processes and fluid-rock interactions, and the original pore structure in the sandstones was extensively modified during burial They have overall ultralow porosity and permeability and small pore throat [2, 3]. It is essential to understand the influence of sandstone composition and texture on variations in diagenetic types and processes This can aid in understanding heterogeneous reservoir evolution and petroleum flow during deep burial, revealing the origin and distribution of effective porosity and permeability in such sandstone reservoirs. (1) To classify sandstone petrofacies and to examine petrographic and petrophysical characteristic of different petrofacies (2) To indicate whether different petrofacies have undertaken different diagenetic pathways (3) Understand the formation of effective porosity and permeability in the heterogeneous deep reservoirs
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