The oolitic limestones are the best reservoirs close to the exposure surface, whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface. The heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution-reprecipitation process induced by thermochemical sulfate reduction, during which small amounts of net porosity are calculated to have been created. Porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution. The higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation, rather than the creation of new pores. During deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss, whereas the dolostones generally retained more porosity and have better connectivity of the micropore network. During the subsequent dolomitization, the early-formed oolite structure were mostly inherited by the dolostones on the eastern side, with fabric-retentive textures in the layers close to the exposure surface and fabric-destructive textures in the layers far from the exposure surface. The layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater, resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement. On both sides of the K-L Trough, eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface. A petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co-evolution of oolite structure and pore network. However, they have contrasting diagenetic histories and reservoir qualities: the sour oolitic dolostone reservoirs on the eastern side of the K-L Trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side. The Ratcliffe lithologies can be divided into two major diagenetic provinces, the near-shore/supratidal, which consists of rocks which have been moderately to highly altered by diagenesis, and the open marine, which is comparatively much less altered.Two oolites of the Lower Triassic Feixianguan Formation on the margins of the Kaijiang-Liangping (K-L) Trough in the NE Sichuan Basin were deposited in the upper parts of similar shallowing-upward sequences, and are capped by a subaerial exposure surface. Fracturing and brecciation also occurred. The most common forms of diagenesis include dolomitization, replacement, recrystallization, cementation, nd dissolution. The facies in the studied cores showed that much diagenesis has occurred in the Ratcliffe interval. Highly saline conditions at the end of the Ratcliffe allowed deposition of large amounts of halite. The environment gradually became more restricted and deposition of large amounts of anhydrite and dolomite occurred around the basin margin. The initial deposition consisted mainly of marine limestones. These facies were deposited in a regressive setting. Study of cross-sections, well core, and thinsections show that the Ratcliffe is comprised of six major facies: 1) brachiopod-bryozoan-echinoderm wackestone/packstone, 2) peloid packstone/wackestone, 3) oolite-peloid packstone, 4) laminated mudstone, 5) quartz siltstone packstone, and 6) anhydrite-dolomite mudstone. It is Mississippian in age and crosscuts the lower Charles and upper Mission Canyon Formations of the Madison Group. The Ratcliffe interval in North Dakota is a log marker-defined unit in the Williston Basin.
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