Impact of secondary processes to the structure of the porous space of the upper Permian carbonate deposits of the northern part of the Pre-Caspian basin

UDK: 551.263
DOI: 10.24887/0028-2448-2021-5-31-34
Key words: Secondary alteration, dolomitization, anhydritization, northern side of the Pre-Caspian basin, Permian deposits, carbonates
Authors: A.R. Ayupov (Gubkin University, RF, Moscow), K.O. Iskaziev (Gubkin University, RF, Moscow), S.F. Khafizov (Gubkin University, RF, Moscow)

The analysis of the facies distribution of reservoirs within the Moscovian-Artinskian sedimentation rim within the northern part of the Pre-Caspian basin was carried out. The Teplovsko-Tokarevskaya group of deposits is a chain of carbonate structures stretched in the sub-latitudinal direction. They are consisting mainly of bioherm structures, which include tubifytes, foraminifera, crinoidea, ostracods, ostracods, etc. From the lithological point of view, the reservoir rocks are represented by dolomite-limestone differences-from organic limestones to secondary chemogenic dolomites. The influence of facies distribution and secondary dolomitization on the structure of the pore space remains controversial and requires detailed study. The concepts of secondary dolomitization were analyzed and one of the concepts of the formation of secondary dolomites and anhydrites was used to justify the facies distribution. Secondary transformation (dolomitization) form a sweet spots zone in the Artinskian carbonate horizon when high-salinity (Mg2+) waters from the Filippovian horizon carbonates are infiltrates to Artinskian carbonate as a result of the discharge of elision waters during digenetic dehydration of gypsum. After the anhydride overlaying of the Artinskian carbonate structure, several regressive-transgressive cycles occurred, this formed a sequence of consistent dolomite-limestone and gypsum layers in the Filippovian time. During the diagenesis water contained in the gypsum was dehydrated into the permeable zones in carbonates of the Filippovian horizon, followed by unloading in the region of the Artinskian horizon. Evaporite sedimentation of chemogenic carbonates and gypsum created a condition for the subsequent infiltration of sulphate and magnesium waters in the direction dip formation angle. The source of magnesium is the water remaining after the precipitation of gypsum and carbonates in the Filippovian time.


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