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Parameter "reservoir-non-reservoir" is the main indicator of productive horizons of the fields with low reservoir porosity and permeability (RPP) (Archegov, Bazanov, 2000).
A highly irregular oil saturation caused by jointing of carbonate terrigenous rocks is characteristic for such deposits. Oil saturation intensity is closely associated with porosity and permeability (porous and fracture). However, low-porous "semi-reservoirs" are also frequently productive (Gmid, 1991).
The author considered the A. Titov (oil) and Suzun (oil-and-gas) fields in the Timan-Pechora and Yenisei-Khatanga oil-and-gas bearing provinces, correspondently.
Productive deposits of A. Titov field are confined to three Lower Devonian beds D1-I, D1-II, and D1-III. Reservoirs are represented by carbonate rocks, predominantly recrystallized secondary dolomites with low porosity 8-12% and permeability indices; however, in some areas of the deposit, powerful oil influxes up to 850 m3/s are recorded. From the diagram of porosity and permeability distribution, we can observe that jointing permeability prevails over the matrix one. Faults can influence the increase of jointing, because open jointing in near-fault zones is 2-3-fold as compared to the background one; in these zones, high-yield oil influxes were obtained (D1-I bed). Dolomitization processes, when numerous faults develop, also influence the RPP increase of carbonate reservoirs. Secondary processes of recrystallization and leaching in dolomite reservoirs (D1-I bed) form secondary porosity and cavernosity observed in D1-II and D1-I deposits.
An event of oil yield increase in terrigenous reservoirs is examined as exemplified by Suzun oil-and-gas field. Lower Cretaceous beds NKh-I and NKh-III-IV of the Lower Kheta Formation are oil bearing. Oil yields vary from 18 to 270 m3/day. NKh-I net thickness varies from 0 to 22.4 m, and NKh -III-IV, from 0 to 8.2 m. Sandstone porosity varies within 11.2-24 %, oil saturation, 55-76%, permeability reaches units.
Dependence diagram of the porosity of productive terrigenous reservoirs and oil yield is compiled based on borehole data on the Suzun field. Despite a decreasing porosity, oil yields increase. Presumably, an increasing jointing of terrigenous reservoirs is the main factor of oil yield growth. The fact that the Suzun area lies in a deep fault belt promotes rock jointing and oil yield increase.
Having considered different fields, we can conclude that for determining areas and deposits with oil-and-gas presence in low-permeable carbonate and terrigenous reservoirs offering the best prospects for exploration and development, it is necessary to reveal zones favourable for jointing and secondary porosity development, the presence of which is confirmed by:
Strike-slip zones;
Zones of process development (leaching, recrystallization).
These diagnostic features of elevated RPP of reservoirs can be also used for other areas and fields.
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