Geology of Sub-Salt Carbonate Rocks in the Rozhkovsky Structure, Northern Precaspian Basin, Kazakhstan: Implications for the Formation of Carbonate Reservoir in a Relatively Closed Diagenetic System

Sub-salt carbonate successions in the Precaspian Basin host several world-famous oil/gas fields along the basin margin. The Early Carboniferous Sub-salt carbonate rocks are associated with the Rozhkovsky structure near the northern margin of the Precaspian Basin. These carbonate successions are dominantly shallow-marine carbonate sediments that are covered with widespread Permian salt deposits. This study contributes to the reservoir preservation in a relatively closed diagenetic system, which developed in a ramp setting during early Carboniferous time. Through detailed petrographic studies of 370 m of cores and 316 rock samples (as thin sections), nine lithofacies were identified within the study area. These carbonates are characterized by grain-dominated limestones, which are interpreted to be deposited in an inner-mid ramp. The assemblages of lithofacies revealed five depositional facies/environments, including: restricted inner ramp, well-circulated inner ramp, high-energy shoal and bank, skeleton mound, and wellcirculated mid-ramp. Sixteen diagenetic features and/or events were recognized in the Sub-salt carbonate rocks, and which are interpreted to have formed in three diagenetic environments: near-surface shallow marine-meteoric; shallow burial (within 600m in depth), and intermediatedeep burial. Cementation and compaction features were relatively common and widespread throughout the rocks, while dolomitization, dissolution, and fracturing occurred locally. The δ18O and δ13C values range from -9.94 to -3.5‰, V-PDB and +2.08 to +6.4‰, V-PDB, respectively, for calcite cements from -5.7 to -4.14‰, V-PDB and +3.51 to +5.2‰, V-PDB, respectively for dolomite. These values are similar to expected values precipitated from the Early Carboniferous seawater, suggesting that they were formed by the residual marine pore waters from the host rocks. The depleted oxygen isotopic values of blocky calcite cements are related to ii the elevated burial temperatures. A fluid inclusions microthermometry study shows the average Th from syntaxial calcite cements is 90°C and that the salinities range from 20 to 24 wt% NaCl. The syntaxial calcite is therefore interpreted to be formed during burial diagenesis rather than the common assumption that it is formed during near-surface marine diagenesis. The distribution and characteristics of porosities and permeability in different lithofacies were analyzed in order to evaluate the quality of the Sub-salt carbonate reservoirs. The initial porosity and permeability are strongly related to lithofacies. The grainstone holds the highest average porosity (6.6%) and permeability (7.1 mD), and are positively correlated. The dolostone has a porosity of 6.8% with a permeability of 6.3 mD. Three equations were established for the porosity calculation-based well logs and calibrated with measured core porosity. These equations can be applied for reservoir quality prediction in the un-cored carbonate successions for hydrocarbon explorations in new frontiers. Contrary to the conventional accepted concept, the dolomitization in the study area does not significantly improve the porosity and permeability in grain-supported limestone lithofacies. In the study area, the occurrence of the carbonate reservoirs is related to grain-dominated lithofacies that are developed along the shallow-water high energy inner ramp setting. The laterstage diagenesis of these carbonate rocks is interpreted to have occurred in a nearly closed system because these carbonate rocks were sealed by the massive salt above, which substantially reduced the degree of burial diagenesis in these carbonate rocks. As a result, the porosity in these Sub-salt carbonate reservoirs are dominantly primary porosities. This is unique for the studied carbonate rocks because carbonate reservoirs, in general, are characterized by significant amount porosity formed during diagenesis.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Doctoral -- first

Discipline thesis:degree_discipline

Geology

Grantor dc:publisher

Faculty of Graduate Studies and Research, University of Regina

Year dc:date.issued

2021

Author dc:creator

• Zhang, Siyang

Advisor dc:contributor.advisor

• Qing, Hairuo

Committee members dc:contributor.committeemember

• Coulson, Ian
• Dale, Janis
• Zeng, Fanhua

Language dc:language.iso

en

DOI

https://doi.org/10.82465/4795

OAI identifier oai:identifier

oai:uregina.scholaris.ca:10294/14461