Reservoir heterogeneity and diagenetic influences in the oil-bearing Lakadong Member of the Upper Assam Basin, India
摘要
The oil-bearing Lakadong Sandstone (Upper Paleocene–Lower Eocene) of the Upper Assam Basin (India) was studied to quantify the effects of diagenetic processes on its reservoir properties. The Lakadong Sandstones, occurring at depths of 3000–4000 m within Oil India Limited’s (OIL) operational areas in Assam, serve as a major hydrocarbon producer despite their complex nature. Wells completed in these reservoirs show contrasting performance: some horizons yield prolific oil production, while others are comparatively poor. To evaluate the factors governing reservoir performance, this study focuses on a comprehensive investigation of mineralogical composition, textural relationships, and diagenetic modifications that have governed the reservoir properties. The average framework composition of Lakadong Sandstone is Q98.03F1.62R0.34, fine- to medium-grained, quartz arenite to subarkose, and quartz wacke. Syndepositional silty to clayey matrix (avg. 2.16%) occurs as pore-filling material. The average grain contact types observed are: floating (47.51%), point (12.80%), long (33.25%), concavo-convex (5.92%), and sutured (0.52%). The diagenetic heterogeneity divided the reservoir into two distinct sequences. The upper to middle sequence consists of clean, tightly packed sandstones with an average optical porosity of 16.55%, while the lower sequence is dominated by extensive, early-stage poikilotopic calcareous cement, with an optical porosity of 11.20%. This early calcareous cementation in the lower part of the sequence inhibited compaction, produced floating grain textures, and is regarded as the primary cause of porosity reduction. The low average Compaction Index (1.26) further confirms the dominance of cementation over compaction. Permeability ranges from 2.31 to 169.64 mD, with higher values observed in the middle and upper sequences. Pore-filling kaolinite (16.90%), along with chlorite (7.82%) and fibrous illite (3.85%), further degrades reservoir quality, whereas dissolution of the unstable grain, along with partial dissolution of carbonate cements, enhances it. The occurrence of glauconite indicates marine influence and seawater interaction. The potential to serve as a reservoir is closely related to the diagenetic alterations in sandstone. To improve hydrocarbon productivity, hydraulic fracturing is recommended for the compaction-dominated upper sequences, whereas matrix acidizing is more suitable for the heavily cemented lower sequences of the Lakadong Sandstone.
Research highlightsExplores how diagenetic processes affect the reservoir quality of the Lakadong Member. Reveals the vertical heterogeneity in reservoir characteristics within the Lakadong Member. Suggests targeted well stimulation approaches for different reservoir intervals influenced by distinct diagenetic processes.