Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

Abstract

This study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.

Growth and Distribution of Coal-Measure Source Rocks in Mixed Platform: A Case Study of Carboniferous in Bamai Area, Southwest Tarim Basin, China

“…The Carboniferous strata are one of the main subjects of oil and gas exploration in the Bamai area and contain the Bachu Formation, Karashayi Formation, and Xiaohaizi Formation from the bottom up. Under the action of dry and wet climate alternation and the rise and fall of relative sea level, a deep-water shelf, shallow-water shelf, lagoon bay, delta, and other sedimentary environments were mainly developed [32][33][34][35][36][37]. The main lithologies include limestone, dolomite, marlstone, mudstone, etc., and these lithologies are partially intercalated with gypsum dolomite and gypsum mudstone and locally embedded with coal seams, gypsum dolomite, and gypsum mudstone [38,39].…”

Diagenetic Fluid and Its Impact on Sandstone Reservoirs in the Southern Boxing Sag, Dongying Depression, Bohai Bay Basin, China

“…18 Geofluids fluctuations in the δEu values, with an average of 1.10, which is larger than the δEu values of mudstones and displays a slightly positive Eu anomaly, which probably reveals that hydrothermal fluids are involved in the precipitation of late carbonate[54]. However, the Th ( °C) of late-stage carbonate ranges between 118 °C and 126 °C, which indicates a normal burial condition and is inconsistent with the hydrothermal fluid source (Figure12).…”

“…However, the origin of fluids, temperature conditions for fluid activity, and transformation time of diagenetic fluids are still debatable [12,13]. The use of major and minor elements together with C-O isotope of carbonate minerals was the most widespread methods to track the source of fluids and the organic-inorganic reactions in siliciclastic rock reservoirs (e.g., [14][15][16][17][18][19]). Fluid inclusions within authigenic minerals were conducted to ascertain the time of fluid combining with the burial history [20][21][22] and provided additional evidence to distinguish the origin of diagenetic fluids [23].…”

“…Moreover, the C-O isotopes of carbonate cements represent the thermal evolution of organic matter and provide additional information about diagenetic fluids[57]. δ 13 C and δ18 O of micritic carbonate within mudstone possibly reflect the carbonate characteristics during the sedimentary period. Their averages are 3.5% and −9.5%, respectively[14].…”

“…Their averages are 3.5% and −9.5%, respectively[14]. The negative shift of δ18 O within late carbonate cement indicates the higher temperature of precipitation because δ18 O is sensitive to the diagenetic temperature[58]. The δ13 C value is deemed an effective index for organic-inorganic interaction in the diagenetic process[59] and is associated with preexisting carbonate dissolution, bacterial sulfate reduction, microbial degradation of organic matter, and thermal decarboxylation of hydrocarbons[60,61].…”