Search

Pore-cave systems formed by karstification in the eogenetic stage of carbonate rocks provide abundant potential reservoir space for hydrocarbons. However, whether these dissolution pore-caves can become effective reservoir spaces during the later burial period, serving as the key to the success of hydrocarbon exploration. Therefore, it is important to explore the fluid activities and their alteration effects on eogenetic karst reservoirs during the later burial. Focusing on the Cambrian Longwangmiao Formation in the northwestern Sichuan Basin, this study systematically analyzed the formation of reservoir space in the eogenetic stage and the reworking of the system by fluids in the later stages, based on petrology, geochemistry, burial history, and tectonic evolution data. Results showed that many millimeters to several centimeters scale of pores and caves in the Longwangmiao Formation were produced by eogenetic karstification. These pore-caves underwent by two episodes of dolomite infilling in the shallow burial stage (D1) and in the Caledonian–Hercynian period (D2). Geochemical parameters indicate that D1 and D2 were both affected by meteoric water. In the early shallow burial stage, the dolomitic fluid was enriched in a relatively closed, reducing environment, whereas in the later stage, the fluid was affected by a relatively open oxidizing environment due to Caledonian–Hercynian fractures. Both D1 and D2 took place before the massive hydrocarbon migration from the Cambrian source rocks in the Middle Permian to those of the Middle Triassic. After the formation of the dissolution pore-caves, the precipitation from two episodes of dolomitic fluids led to the degradation of the Longwangmiao Formation carbonate reservoir space in the northwestern Sichuan Basin. In the southern part of the Shatan section-Well MS1, closed to the paleo-uplift of the central Sichuan Basin, where eogenetic karstification was superimposed by Caledonian–Hercynian supergene karstification, may be form effective reservoir and is a significant prospect for exploration. [ABSTRACT FROM AUTHOR]

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation. [ABSTRACT FROM AUTHOR]

Understanding the processes and products of carbonate diagenesis is crucial to the exploration and optimum development of palaeokarst hydrocarbon reservoirs in Permian carbonate rocks. Several well-documented outcrops and well drills in the Permian marine carbonate strata around the Sichuan Basin, South China, encompassing well-developed paleokarst landforms in the Middle-Upper Maokou Formation have been investigated. The sections and cores recording paleokarst reservoirs exhibit very similar carbonate diagenetic products and processes from various diagenetic environments by much most cementation and formation of secondary porosity in carbonates. The major diagenetic environments experience the zones of seawater, surface or near-surface atmospheric water, and subsurface mixed water between marine and meteoric. Characteristic diagenetic textures produced in each environment are described and different generations of calcite cements related to the process are responsible for their formation by sedimentary petrology and stable isotope geochemistry. Paleokarst under the constrained condition of surface and near-surface atmospheric water is the most favorable for oil-gas accumulation and presents some karst-related pores-caves-fissures with less or no fillings. Dissolution and corrosion of carbonate by atmospheric freshwater can alter the accumulation spaces and develop better fractures-caves system. These characteristic could exhibit some substantial data for better predicting porosity trends in Permian carbonate rocks and also provide important geological information for further exploring oil-gas resource in marine carbonate strata. [ABSTRACT FROM AUTHOR]