Diagenesis of Basal Sands of The Cretaceous Lower Goru Formation of The Badin Area, Sindh, Pakistan
The basal sands of Lower Goru Formation are reservoirs for over twenty (20) fields of the Badin Platform which is the eastern part of Sindh Platform of Indian Shield. They are divided into the Upper and the Lower Basal Sands separated by Talhar Shale.
Thirty seven core samples were selected from seven wells reaching 5000ft to study the reservoir characteristics of the Lower Basal Sands in the Badin Area. Thirteen samples were poorly sorted, fine-grained sandstones with grain size less than 0.5mm. The other fourteen samples were well sorted, medium grained sands of 0.5 mm to 1 mm grain size. The remaining ten showed little pore spaces 1-5% porosity between the mosaics of the quartz grains. Destruction of the original porosity was mainly due to intense phase of quartz overgrowth during the diagenetic stage. The precipitation of the authigenic clay from dissolution of feldspar minerals have also acted towards reduction in porosity.
Appreciable secondary porosity 5-10% was observed in only four samples. Partial dissolution of unstable grains of feldspar resulted in intragranular porosity. Local complete dissolution resulted in moldic porosity. The two processes produced isolated pore spaces which did not affect imparting permeability to the rock. Vuggy porosity was formed by the leaching of framework grains and some matrix material. Where this type of porosity is present, the sandstones form tight reservoirs for gas. Well completion and stimulation may require the hydraulic fracturing of these tight sandstones to produce economic flow rates.
The entire basal sands were deposited in marginal and shallow marine environment. The presence of coal, carbonaceous mud and siderite/glauconite in the sands reflects the intimate marine/non-marine intercalation probably the result of tidal activity.
Lower Goru is a tight sand reservoir. It has produced and will continue to produce with proper fracturing job.