Print this pagePrediction of the Spatial Variability of Diagenesis and Porosity Using Sequence Stratigraphy in Middle Carboniferous carbonates from Southern Kazakhstan: Implications for North Caspian Basin Hydrocarbon Reservoirs of the CIS
FRETWELL, P. NICK., DAVE HUNT, DAVID CRAIK, HARRY E. COOK, PATRICK J. LEHMAN N, WILLIAM G. ZEMPOLICH, VITISCHLAV ZHEMCHUZHNIKOV and VALYA ZHAIMINA
Field studies of Mississippian to earliest Pennsylvanian (Visean-Bashkirian) shelf carbonates in the Bolshoi Karatau mountains of southern Kazakhstan show strongly cyclic shallowing-upwards sequences on a scale of 0.35 to 25 metres. These relative sea-level fluctuations are associated with palaeosol formation and influence cement and porosity distribution.
Porosity is destroyed at and 1-2m below palaeosol boundaries because of precipitation of early marine calcite and dolomite (now dedolomite) and silica cements. Dissolutional porosity was created during palaeosol formation at the sequence boundaries, but has been destroyed by calcite and dedolomite cements. Zones of meteoric calcite (some filling fenestral porosity) are correlatable by cathodoluminescence across at least 5 sequence boundaries (>70m), suggesting development of a major meteoric phreatic system during prolonged exposure. {Delta}18O of the later zones range between -7 and -11 per mil (PDB) indicating precipitation from warm fluids.
Replacive dolomite occurs in the mid to lower parts of Middle Visean sequences, associated with stylolites, but also forms extensive dolostones. Dolomite in the Upper Visean is less common and unrelated to depositional sequences during highstands, but is more common in lagoonal transgressive deposits. Stylolite-related dolomite ranges between delta{18}O of -10 to -11, and its {87}Sr/{86}Sr values are consistently higher (0.7084-0.7094) than those for contemporary depositional calcite and marine cements (0.7079-0.7081). This supports a warm, post-compactional subsurface origin.
Most porosity in the Boisho Karatau Visean is secondary after dolomite, produced by post-Cretaceous weathering.