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The Upper Cretaceous rocks of Lidam Formation are an important hydrocarbon reservoir in SE Sirt Basin, Libya. Although these rocks are considered to be prodective regionally, but locally they exhibit variable recovery efficiencies, which may be deeply affected by vertical and horizontal heterogeneity of porosity and permeability. The particular nature of this heterogeneity is still poorly known. The methodology of analyzing outcrops to be correlated to the subsurface reservoir rocks is becoming more and more a common practice allowing to study and then model the lateral variability of reservoir rocks. In this study the Ain Tobi Member, Upper Cretaceous age exposed at Abu Ghaylan, Gharyan Area, have been selected as analog for Lidam Formation of "PT" Field, SE Sirt Basin, Libya. Outcrop analog offered an exceptional opportunity to examine the petrophysical heterogeneity and facies changes which could help to develop and improve the suggested models for understanding subsurface reservoir trends. The detailed investigations of identified Lidam and Ain Tobi facies exhibit very similar depositional environments. Both were mostly deposited in tidal flat, subtidal lagoon, shoal, and/or open shelf depositional environments within ramp setting. Limited differences in lithology, such as common anhydrite present in Lidam units, could probably related to paleogeography, and the degree of restriction and/or the spatial location of Ain Tobi studied sections within the ramp setting. Eight facies are identified of Lidam studied units; (1) silty sandstone, (2) alternating anhydrite and stromatolite, (3) dolocrete, (4) dolo mud/wackestone, (5) shale, (6) peloidal wacke/packstone, (7) peloidal pack/grainstone, (8) and planktonic foraminifera mud/wackestone facies. On the other hand, eleven facies are identified of Ain Tobi units; (1) sandy dolo-mudstone, (2) dolo-stromatolites (3) alternating dolo-mudstone and dolo-packstone (4) dolo-mudstone (5) macro gastropod dolo mud/wackestone (6) bioturbated-mottled dolo-mud/wackestone (7) clay (8) marl (9) dolo wacke-packstone (10) cross-stratification dolo-pack/grainstone (11) dolo-float//rudstone facies. The most important diagenetic processes recorded in Lidam and Ain Tobi Member include micritization, dissolution, dolomitization, and cementation.
Dedolomitization processes are very common and clearly observed in most Ain Tobi samples, but not recognized in Lidam studied units. The compaction stylolite structures in Lidam studied samples are well recognized in studied cores and thin-sections, but not verified in Ain Tobi studied samples. Dolomitization and cementation diagenetic processes of Lidam and Ain Tobi studied units played an important role in reservoir characteristics. Seepage reflux dolomitization model suggested for Lidam and Ain Tobi studied area show seaward obvious increase in dolomite crystal size together with the enhancement of reservoir properties. Anhydrite and dolomite cements concentrated in landward Lidam tidal flat and proximal lagoonal subtidal facies, whereas calcite cement was very common in Lidam shoal facies. Late calcite and overgrowth dolomite cements destroyed the most secondary porosity developed in Ain Tobi studied units. Three depositional sequence are recognized in Lidam studied wells. Depositional sequence 1-L is characterized by high energy shoal deposits overlain by lagoonal facies. Depositional sequence 2-L is characterized by open shelf deposits overlain by lagoonal and tidal flat deposits. Depositional sequence 3-L is characterized by lagoonal deposits overlain by tidal flat deposits. Lidam studied wells show obvious shallowing upward trend, where outer ramp and middle ramp deposits overlain by inner ramp deposits. Three depositional sequences are identified in Ain Tobi studied units, where depositional sequences 1-A and 2-A are mostly built by pertidal cycles, which are dominated by cycle-capping subaerial exposure features. Depositional sequence 3-A characterized by rudist deposits overlain by lagoonal and high energy shoal facies. Petrophysical analysis of Lidam and Ain Tobi plugs show that dolomitized lagoonal subtidal facies exhibit better reservoir properties in contrast with tidal flat and shoal facies. Lidam and Ain Tobi lagoonal subtidal facies also show variations of porosity and permeability. This variations were examined by more than 40 plugs taken from Ain Tobi lagoonal subtidal horizontal bed along 500 m. The plugs of this horizontal bed show a relative increase of porosity and permeability toward the center of lagoon, which is very consistent with size increase of dolomite crystals illustrated in dolomitization model for Lidam and Ain Tobi studied units. Lidam reservoir heterogeneity is more understood and interpreted by Ain Tobi outcrop analogs which is very essential to provide accurate descriptions of lateral variability in reservoir dolomite rock fabrics. The integration of sedimentological studies, sequence stratigraphy, and petrophysical analysis of Lidam and its time-equivalent Ain Tobi Member can relatively draw the porosity and permeability trends of Lidam carbonate reservoir.
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