Sedimentological cyclicity and lithological features of the Kashirskian sequence in the northwestern Bashkortostan

UDK: 553.98

DOI: 10.24887/0028-2448-2020-7-79-81

Key words: cyclicity, facies, limestone, dolomite, reservoir properties, Kashirskian sequence

Authors: R.V. Mirnov (RN-BashNIPIneft LLC, RF, Ufa)

The Kashirskian sequences of Moscovian stage of Middle Carboniferous series are oil-and-gas-bearing deposits in the Republic of Bashkortostan, but they remain underinvestigated. The most of oil deposits are concentrated in the northwest part of republic. Oil flow rates of single well sometimes rich 50 CMPD and more, but often there are water influx of wells in close proximity. At present time there are not identificated any patterns of reservoirs and seal-rocks development area. There is a problem with oil-saturated reservoir-rocks allocation as a result of geophysical data interpretation. The Kashirskian sequence contains a large amount of dolomite, but the questions of its genesis, patterns of its development area and relationships with reservoir properties are controversial. Kashirskian sequence has a cyclic structure which is typical for epeiric seas. The survey resulted in the identification of elementary “shallowing upward” sequences with 3-8 meters thickness. They can be merge into sequences of higher range, visible on the log curves. The composition of idealized Kashirskian sequence is given below. Low-porosity bioturbated wackstones of subtidal zone is at the bottom. Above are intertidal bioclastic grainstones with high porosity and permeability. The upper member of the sequence is microcrystalline dolomites with high porosity (10–25% and more) and low permeability. The research is aimed to study various levels of cyclicity, lithological features of each part of cyclothem and its relationships with reservoir properties. Layers of paleosoils on the top of sequences mark breaks in sedimentation. Analysis shows different types of reservoirs distribution in the section. Probably, water influxes in some wells related to lenticular bodies of microporous dolomites with high residual water saturation. It’s necessary to conduct thin-layer correlation, based on wells with core and cuttings and enhanced well-logging methods to define development areas of grainstones and microporous dolomites. It will help to create reliably geological framework and improve drill efficiency. References 1. Burikova T.V., Savel'eva E.N., Husainova A.M. et al., Lithological and petrophysical characterization of Middle Carboniferous carbonates (a case study from north-western oil fields of Bashkortostan) (In Russ.), Neftjanoe hozjajstvo = Oil Industry, 2017, no. 10, pp. 18–21. 2. Van Lith Y., Warthmann R., Vasconcelos G., Mckenzie J.A., Microbial fossilization in carbonate sediments: a result of bacterial surface involvement in dolomite precipitation, Sedimentology, 2003, V. 50, pp. 237–245. 3. Kirkham A., Patterned dolomites: microbial origins and clues to vanished evaporates in the Arab Formation, Upper Jurassic, Arabian Gulf, London: Geological Society, 2004, V. 235, pp. 301–308. 4. Samylina O.S., Zaytseva L.V., Sinetova M.A., Participation of algal–bacterial community in the formation of modern stromatolites in Cock Soda Lake, Altai Region (In Russ.), Paleontologicheskiy zhurnal = Paleontological Journal, 2016, no. 6, pp. 92–101.