Origin of carbonate cements and reservoir evolution of tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, China

Origin of Grain-Coating Chlorite and Implications for Reservoir Quality in the Triassic Deep-Buried Volcaniclastic Sandstones, Central Junggar Basin, Northwestern China

“…The grain-coating chlorite presents the characteristic of isopachous and small thickness, with poor shape and disorganized crystals (Figure E,F), suggesting a potential transformation from precursor clay . The growth of grain-coating chlorite requires large quantities of ions, including Fe and Mg, which are commonly derived from the hydrolysis and alteration of volcanic materials and heavy minerals. ,,, Hydrolysis and alteration of intermediately basic volcanic rock fragments in the Kelamayi Formation sandstones provide large quantities of Fe 2+ , Mg 2+ , and other cations for the reaction, a result supported by the in situ precipitation of chlorite from clast alteration (Figure F and Figure B,F).…”

“…Clay grain coatings are known as one of the key factors in preserving porosity. , Grain-coating chlorites are probably the most common grain coating that has been found to be closely associated with the quality of deeply buried clastic reservoirs. ,− Grain-coating chlorite can improve the reservoir quality by inhibiting quartz cementation. − Previous studies have shown that in deeply buried sandstones, grain-coating chlorites of appropriate thickness and high coverage maintain higher porosities. ,− However, too thick a grain-coating chlorite can reduce the pore throat radius, thereby diminishing reservoir properties by clogging. ,− Consequently, the positive and negative impacts of grain-coating chlorite on reservoir quality have attracted widespread attention. , The formation mechanism of grain-coating chlorite is quite intricate. Some propose that under conditions of pore fluids rich in Fe and Mg during diagenesis, grain-coating chlorite can develop through precursor-driven recrystallization induced by heat or through in situ transformation and alteration of precursors, and grain-coating chlorite formed by these two mechanisms exhibits distinct morphological differences .…”

“…Clay grain coatings are known as one of the key factors in preserving porosity. 10 , 12 Grain-coating chlorites are probably the most common grain coating that has been found to be closely associated with the quality of deeply buried clastic reservoirs. 6 , 13 − 15 Grain-coating chlorite can improve the reservoir quality by inhibiting quartz cementation.…”

Residual Oil Distribution Pattern in a Fault-Solution Carbonate Reservoir and Countermeasures to Improve Oil Development Effectiveness

“…Loft-type residual oil refers to the residual oil that cannot be directly produced from the fractured-vuggy reservoir above the top of the production interval of the oil well. In the production well, above the oil-water interface of the fractured-vuggy in the production interval, with the production of crude oil, the oil-water interface in the lower part of the reservoir continues to rise [26]. As the oil-water interface rises to the production interval, flooding occurs in the production interval of the oil well, resulting in the presence of attic-type residual oil in the upper fissures that were difficult to recover, as revealed in L18CX, L14, and L13CH wells.…”

Role played by clay content in controlling reservoir quality of submarine fan system, Forties Sandstone Member, Central Graben, North Sea

“…The origin of carbonate cements can be established based on carbon isotopic compositions of the cements (Irwin et al, 1977;Ma et al, 2017;Mao et al, 2019;Stewart, 1995). The ẟ 13 C values for the carbonate cements in the sandstones range from -21.12 ‰ to +9.48 ‰ (average -7.04 ‰; Table 2; Figure 18), suggesting a wide range of sources for carbon including both organic and inorganic sources (Figure 18).…”