Abstract The Lower Cretaceous Bashijiqike Formation in Kuqa depression belongs to a continental red bed sandstone with poor reservoir quality, and is deeply buried to 5.5–7.0 km but some successions can host porosity >10% and permeability larger than 10 mD. Carbonate cements are important pore-filling constituents, and they control the reservoir quality and heterogeneity of Bashijiqike sandstones. Red hematite-bearing grain-coating clay minerals inhibit the pore occluding quartz cements and preserve intergranular pores. Here we investigate the ability of grain-coating clays to inhibit carbonate cements. Thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), and cathode luminescence (CL) analysis were performed to investigate the petrography, pore systems, diagenesis and diagenetic minerals of the red bed sandstones. The results show the hematite-bearing grain coatings give the ‘‘red bed’’ colors of the sandstones. The sandstones are heavily mechanically compacted, and the predominant pore-filling cements include carbonates and clay minerals, while quartz cements are volumetrically not important in reducing porosity. The grain-coating clays are mainly hematite-bearing mixed-layer illite and smectite. The paragenetic sequences of various types and generations of diagenetic events were reconstructed. Sandstones containing effective mixed-layer illite and smectite coatings, in which the intergranular pores can be preserved, have high porosities. Carbonate cement is never found in sandstones with effective grain coating clays, and carbonate cement precipitates where there are no sufficiently thick and continuous grain coats. The clay coatings retard or inhibit eogenetic calcite cements and progressively restrict late-stage carbonate (dolomite) cementation during burial. The hematite-bearing grain-coating clays help the red bed sandstones retain good reservoir quality at great depths.

The Abu Gabra and Bentiu formations are widely distributed within the interior Muglad Basin. Recently, much attention has been paid to study, evaluate and characterize the Abu Gabra Formation as a proven reservoir in Muglad Basin. However, few studies have been documented on the Bentiu Formation which is the main oil/gas reservoir within the basin. Therefore, 33 core samples of the Great Moga and Keyi oilfields (NE Muglad Basin) were selected to characterize the Bentiu Formation reservoir using sedimentological and petrophysical analyses. The aim of the study is to de-risk exploration activities and improve success rate. Compositional and textural analyses revealed two main facies groups: coarse to-medium grained sandstone (braided channel deposits) and fine grained sandstone (floodplain and crevasse splay channel deposits). The coarse to-medium grained sandstone has porosity and permeability values within the range of 19.6% to 32.0% and 1825.6 mD to 8358.0 mD respectively. On the other hand, the fine grained clay-rich facies displays poor reservoir quality as indicated by porosity and permeability ranging from 1.0 to 6.0% and 2.5 to 10.0 mD respectively. A number of varied processes were identified controlling the reservoir quality of the studies samples. Porosity and permeability were enhanced by the dissolution of feldspars and micas, while presence of detrital clays, kaolinite precipitation, iron oxides precipitation, siderite, quartz overgrowths and pyrite cement played negative role on the reservoir quality. Intensity of the observed quartz overgrowth increases with burial depth. At great depths, a variability in grain contact types are recorded suggesting conditions of moderate to-high compactions. Furthermore, scanning electron microscopy revealed presence of micropores which have the tendency of affecting the fluid flow properties in the Bentiu Formation sandstone. These evidences indicate that the Bentiu Formation petroleum reservoir quality is primarily inhibited by grain size, total clay content, compaction and cementation. Thus, special attention should be paid to these inhibiting factors to reduce risk in petroleum exploration within the area.

Petrographic studies of Upper Mississippian Springer and Lower Pennsylvanian (Morrowan) sandstones in six cores from the southeastern Anadarko basin, Caddo and Grady Counties, Oklahoma, reveal a complex diagenetic history that led to the destruction of much primary intergranular porosity. The Springer and lower Morrowan sandstones form prolific oil and gas reservoirs, despite the fine-grained nature of the rocks, the growth of authigenic clays, extensive cementation by quartz overgrowths and carbonate minerals, and burial depths of 11,500-14,800 ft. More than any other factors, the diagenetic creation and preservation of porosity are the major geologic controls on hydrocarbon production from these sandstones. Thin-section petrography and scanning electron microscopy show that porous intervals were formed mainly by extensive dissolution and leaching of detrital grains and authigenic cements. Locally, however, appreciable primary porosity was preserved in Cunningham (Springer Formation) and Primrose (Morrowan) sandstones (as much as 20% in one sample of Primrose sandstone) by the formation of chlorite grain coats on detrital quartz during the early stages of burial and diagenesis. The chlorite grain coats inhibited the occlusion of pore space by preventing pervasive cementation of the rocks by quartz overgrowths. Cross-plots of porosity versus the abundance of authigenic quartz and grain-coating chlorite documentmore » the relationship in two of the cores.« less

Summary Chlorite grain coatings play a significant role in reservoir quality and are widely distributed in Meso-Cenozoic clastic reservoir of several petroliferous basins in china. Chlorite coatings can preserve porosity in deeply buried reservoir by inhibiting quartz overgrowths. However the origin of chlorite coatings in onshore litharenite and their impact on reservoir quality are not well known. This study took the late Permian Upper Urho Formation (P3w) at Western Margin of Junggar Basin, China as an example for understanding the grain-coating chlorite origin and the effect on reservoir quality. The Upper Urho sandstones are mostly litharenite and rich in volcanic rock fragments (VRF). The petrographic and chemical study revealed that the chlorite coatings which formed during eodiagenetic stage at temperature lower than 70°C are Fe-rich chlorite and composed of two superimposed layers. The Eodiagenetic I/S coatings are speculated as the precursors of chlorite coatings. The dissolution of the ferromagnesian rock fragment provided cations needed for chlorite growth. The small content of quartz in P3w sandstones limits the importance of chlorite coatings on preservation of porosity by inhibiting quartz cementation. However the occurrence of chlorite coatings in P3w sandstones acts as a barometer of good quality reservoir intervals.

Non‐marine sandstones in the Upper Triassic Xujiahe Formation form an important reservoir unit in the Sichuan Basin, western China. Widespread authigenic chlorite cement is present in the second and fourth members of the formation, referred to as the Xu II and IV members. This study, based on data from 12 wells from three gasfields, focusses on chlorite development and evolution and attempts to evaluate its influence on reservoir quality. Data included results of X‐ray diffraction (XRD) and electron microprobe analyses (EMPA), scanning electron microscopy (SEM), and stable carbon and oxygen isotope analyses. The results showed that chlorite coatings preserve primary pore spaces in the sandstones by inhibiting the nucleation of quartz cements on grain surfaces. Iron for chlorite growth came from the dissolution of Fe‐rich magmatic rocks derived from the palaeo‐Longmenshan fold‐belt, from dissolved and colloidal iron which had been precipitated in the Xujiahe Formation in a fluvio‐deltaic environment, and from compacting mudstones in other pars of the formation. The chlorite initially precipitated from pore waters during eodiagenesis (< 70 °C; < 2 km burial depth). During mesodiagenesis (>70 °C, > 2 km), the chlorite was modified due to exposure to Mg‐rich fluids derived from the underlying carbonate‐dominated Leikoupo Formation. Thus the Mg content of chlorite in the Xu II member increases with depth. However this phenomenon was not observed in the Xu IV member. The effects of

Abstract Among different diagenetic alterations, quartz cements are the foremost porosity and permeability destroying cement in deeply buried (>2 km) sandstones. Clay minerals are also known to commonly reduce reservoir quality of sandstones, however, detailed diagenetic studies, has suggested that in some diagenetic situations, the authigenic clay minerals not only do not reduce the reservoir quality, but they can help in preserving primary porosity and permeability of sandstones. Based on this research, if during the eodiagenesis, the clay minerals, occur as well-formed, thick, and continuous clay coatings on grains, they inhibit formation of quartz cements, especially overgrowths, during mesodiagenesis. This results in preserving primary porosity and permeability, leading to high reservoir quality in deeply buried sandstones. Different studies show that among different authigenic clay minerals, chlorite is the first most and illite is the second most abundant and important clay minerals in sandstones. Investigation about the conditions of formation and extension of clay coating minerals in sandstones help us in prediction and recognition of strata with high reservoir quality for hydrocarbon exploration. Keywords : Diagenesis; Sandstones reservoir quality; Grain coating clay minerals; Chlorite; Illite Introduction Prediction of the reservoir quality based on sedimentary and diagenetic processes within sedimentary basins is a curtail component for hydrocarbon exploration and