Later fluid alteration of eogenetic karst spaces in carbonate: insights from the Cambrian Longwangmiao Formation, Northwestern Sichuan Basin, China

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 signific

Influence of fluvial crevasse-splay deposits on sandbody connectivity: Lessons from geological analogues and stochastic modelling

To investigate the importance of crevasse-splay elements for the connectivity of fluvial sandbodies, an integrated study has been undertaken that combines data from outcropping and subsurface fluvial successions, and modern rivers, with outputs from forward stratigraphic models and object-based modelling. Literature-derived analogue data on over 2100 crevasse-splay elements from many fluvial systems have been synthesized to obtain quantifications of the proportion and geometry of crevasse-splay units, and of their topological relationships with channel deposits. Some of the analogue data are used to inform inputs to stochastic modelling that is applied to determine how the presence of crevasse-splay deposits affects the connectivity of successions in which channel sandstones are preserved as channelized bodies or compartmentalized point-bar units. Architectural configurations are documented that are variably characterized by (i) crevasse-splay sandbody compartmentalization, (ii) channel-sandbody connections that increase sandbody size and the likelihood of borehole connectivity, and (iii) intercommunication between different channel sandbodies. Analyses of analogue data demonstrate scaling relationships between the morphometry of crevasse-splay elements and associated channel elements and river catchments. Results also demonstrate how the proportions of channel and crevasse-splay deposits tend to covary in stratigraphic volumes. This information, when ported to stochastic models, yields results showing how the influence of crevasse-splay elements on static connectivity varies – in relative terms – as a function of architectural style, net-to-gross ratio and channel-sandbody geometry. Channel-body connections may be more important for smaller and relatively sand-poor fluvial systems, whereas mud-plug compartmentalization of splay elements may be more important for successions with larger channel-body width-to-thickness aspect ratios and lower net-to-gross ratios. The results of this work can be applied to facilitate and improve the characterization and modelling of the static connectivity of subsurface successions that include crevasse-splay deposits. The results also reveal the possible influence of certain geological controls (channel avulsion, river-system scale, overbank cannibalization) on static connectivity