Combined effects of depositional and diagenetic processes on the distribution of rock types in the Lucia petrophysical classification system: A case study of the Dalan and Kangan formations

Abstract

Abstract Upper Dalan and Kangan carbonate formations with Late Permian–Early Triassic ages are considered the main gas reservoir rocks in some Persian Gulf fields. In this study, to investigate the factors affecting the distribution of reservoir rock types, microfacies and diagenetic processes in the upper part of Dalan and Kangan formations in one of the Persian Gulf fields have been investigated. A total of 1110 thin sections and 840 porosity and permeability data (RCAL data) were used. In the petrographic studies, 15 microfacies were classified into four facies belts of the tidal zone, lagoon, shoal, and open marine in an epeiric carbonate platform environment. Studies showed that diagenetic imprints such as calcite cementation, dolomitization, anhydrite nodule formation, dissolution, and compaction had been occurred in three marine, meteoric, and burial diagenetic environments. To investigate and group the reservoir rock types on the Lucia petrophysical diagram, the porosity and permeability data were plotted on this diagram. Depending on the type of microfacies and diagenetic processes, porosity and permeability data are included in all parts of the diagram. This study shows that the reservoir rock of the studied field is very heterogeneous in terms of reservoir properties and is affected by sedimentary and diagenetic processes. Accordingly, grain supported samples have been moved from classes one and two to the lower parts of the diagram, in the non-reservoir section, due to calcite and anhydrite cement. In contrast, in some cases, dissolution and dolomitization in mud-supported microfacies led to data placement from class three of Lucia to classes one and two. Dissolution and dolomitization in grain-supported microfacies have resulted in the placement of these samples in the upper parts of classes one and two. Keywords:Upper Dalan Formation, Kangan Formation, Microfacies, Diagenesis, Lucia diagram Introduction Diagenetic processes are one of the main reservoir quality controllers in carbonate reservoirs in the world. These processes play a constructive or destructive role in the reservoir. They could also have no effect on the initial reservoir quality of the facies (Ahr 2008). The most important application of the geological rock typing method is to investigate the effect of geological factors on petrophysical behavior (Nazari et al. 2019). The petrophysical characteristics in Dalan and Kangan carbonate formations are under the control of sedimentary and diagenetic processes (Nazemi et al. 2019). One of the important methods for rock typing that connects geology and petrophysics is the Lucia method. The aim of this study is to investigate the effect of sedimentary and diagenesis processes on changing the reservoir quality of microfacies on the Lucia petrophysical diagram. Material & Methods For the present study, 242.7 m of the succession, including 1110 thin sections with distances of approximately 20 cm from the upper Dalan and Kangan formations were investigated from well X of one of the fields located in the east of the Persian Gulf. No thin section was available from the K4 unit. To determine the amount of calcite and dolomite, all thin sections were stained with Alizarin Red-S solution. Types of microfacies were described and named by the Dunham method (Dunham 1962). Flügel classification (Flügel 2010) was used to determine the sedimentary environments. Lucia petrophysical chart (Lucia 1995) with 840 porosity and permeability data (RCAL data) were used to investigate the reservoir characteristics of microfacies and the diagenetic processes affecting them. The frequency of each microfacies was determined in Lucia petrophysical classes. Discussion of Results & Conclusions Facies analysis led to the identification of 15 microfacies (MF1 to MF15) in the four facies belts (tidal zone, lagoon, shoal, and open marine) deposited in an epeiric carbonate platform. Studies showed that diagenetic imprints such as calcite cementation, dolomitization, anhydrite nodule formation, dissolution and compaction have occurred in three marine, meteoric, and burial diagenetic environments. In the upper Dalan and Kangan formations, moldic, fenestral, interparticle, intraparticle, intercrystalline and vuggy porosity are observed; but common porosities are moldic, intraparticle and vuggy. Sedimentary and diagenetic processes control the porosity and permeability of the rocks. Depending on the type of microfacies and diagenetic processes, porosity and permeability data are included in all parts of the diagram. An important factor in the distribution of porosity and permeability in carbonate reservoirs is diagenesis (Lucia 2007, Ahr 2008). Due to diagenesis, porosity can enter from one group to another. Dolomitization and dissolution have a positive effect while calcite cement, anhydrite nodule formation, and compaction have a negative effect on reservoir quality. Microfacies that have a higher frequency percentage in classes one and two have a better reservoir quality.

Keywords

• upper dalan formation
• kangan formation
• microfacies
• diagenesis
• lucia diagram