Diagenetic history of the proterozoic carbonates and its role in the oil field development in the Baikit Anteclise, Southwestern Siberia

Abstract

The Meso- to Neoproterozoic Kamo Group and Ediacaran carbonate rocks of the Baikit Anteclise, southwestern Siberian platform, Russia, were studied to reconstruct the history of development of the reservoir rocks and oil field. The Kamo Group is composed of 3.5 to 4 km thick dolostone and, rarely, limestone succession that was deformed before the Ediacaran time. Post-depositional processes played the key role in forming the reservoir properties of the Kamo Group. During the stage of deposition and subsidence (1500–720 Ma) silicification, cementation, and recrystallization caused development of a dense and low-permeability matrix. Pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments, including increase in Mn (up to 0.093%) and Fe (up to 3.11%) contents and 87Sr/86Sr ratios (up to 0.7239), decrease in Sr concentrations (down to 11 ppm), and the development of flat REE-normalized patterns. Folding and uplift in the Baikit basin began about 720 Ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot (70–130 °C) and highly saline (up to 28.7 wt%) fluids.Geochemical (e.g. PAAS-normalized patterns of REE) and isotopic (87Sr/86Sr, δ18O, and δ13C) signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks. Uplift caused subaerial erosion, intense dissolution, and karstification, which penetrated down to 200–300 m below the surface. Folding, uplift, and peneplenization were important for porosity and permeability development in the dolostones of the Kamo Group. The cap rocks were subsequently formed during the Ediacaran and Paleozoic sedimentation cycle.The oil field developed in the Paleozoic. Oil produced from the source rocks of the Madra, Vedreshev, and Iremeken formations of the Kamo Group was dissipated during the uplift and erosion before the Ediacaran sedimentation. After deposition of the Ediacaran and Cambrian strata, source rocks of the Iremeken Formation might have released oil when they passed the oil window, but most of oil in the Kuyumba field was derived from younger Ediacaran source rocks and migrated through fault network from the flanks (the Kureika and Cis-Sayan-Yenisey syneclises) into the reservoir of the Baikit Anteclise that was a topographic high at that time.