IMPLICATIONS FOR PRIMARY DEPLETION OF CARBONATE SLOPE DEPOSITS FROM TENGIZ OIL FIELD AND RESERVOIR CHARACTERIZATION BASED ON AN OUTCROP ANALOGUE

Abstract

This study examines the Paleozoic platform outcrop in the Bolshoi Karatau as an analogue for the giant oil fields in the Pricaspian basin, with a focus on the carbonate distal slope-basin deposits at the Akuyuk section. A detailed sedimentary log (39.6 m thick) reveals a predominance of grainstone lithofacies, consisting mainly of micro- to mm-scale skeletal fragments, microfossils. Notably, occurrences of rugose corals and cephalopod fragments were located mostly on the top of the individual beds, which are settled down. Microfacies analysis divides the section into biosparite (grainstone) and biomicrite (packstone), predominantly comprising microfossils (foraminifers, calcipheres), peloids, and occasionally intraclasts. Deposition occurred in a shallow marine environment and transported through turbidites to the lower slope. Gamma-ray spectrometry analysis indicates very low radioactivity, suggesting predominantly pure carbonates and a marine depositional setting. The proposed main depositional environment is a proximal lobe with mostly amalgamated turbidites, occasionally with muddier parts (packstones). Observations of not full Bouma sequences (only Ta or Ta and Te) (Bouma et al. 1962), mainly Ta (high concentrated flow without any visible sedimentary structures), and chertified trace fossils (thalassinoides) suggest episodic possible Te preservation. Thalassinoides accumulation in soft sediments obscures sediment appearance. The study conducted six scanlines primarily in the lower part of the section, revealing a predominance of extensional fractures filled with calcite, perpendicular to bedding, and stylolites parallel to bedding, indicative of tectonic influence. Analysis of fracture characteristics such as aperture, height, and spacing showed distributions skewed towards smaller values, because of measurement limitations for smaller fractures. The mode and standard deviation for aperture were 0.15 mm and 1.6 mm, respectively, while for height, they were 0.16 m and 0.38 m. Relationships between fracture parameters indicated independence, suggesting complex control factors. The association of veins with faults or folds suggests formation during mountain-building events, contributing to the dynamic geological history of the study area. Further analysis reveals a consistent pattern of densely spaced fractures within individual beds, indicating a strong influence of each layer's mechanical properties. However, variability in fracture heights and spacing suggests structural complexity. While dense spacing and bed confinement are prevalent, and variations highlight the nuanced impact of mechanical stratigraphy on fracture propagation. In a primary depletion analytical model, outcrop sedimentological and fracture data serve as primary inputs, facilitating fluid transport between the matrix and fractures. This model examines how variables such as Vf/Vm and kf/km (sensitivity analysis) affect reservoir depletion, emphasizing rapid fracture depletion compared to constant matrix pressure. Variations in both volume coefficients and permeability coefficients significantly influence the flow behavior in natural fractured reservoirs: higher permeability coefficients lead to faster rate decline, and lower volume coefficients enhance this effect, promoting a rapid transition to fracture matrix flow. The integration of sedimentological logging, microfacies analysis, gamma-ray spectrometry, fracture analysis, and analytical primary depletion modeling sheds light on the importance of field data in understanding fluid flow within naturally fractured reservoirs. These findings offer valuable insights for enhancing reservoir modeling.