Evaluation of a Distinct Sub-Play for Enhanced Exploration in an Emerging Petroleum Province, Bannu-Kohat Sub-Basin, Pakistan

Nadeem, Ahmad ¹; Khan, Moin R.²
(1)Pakistan Petroleum Ltd., Islamabad, Pakistan. (2) Pakistan Petroleum Ltd., Karachi, Pakistan.

A prolific petroleum province, Bannu-Kohat sub-basin, has recently emerged in the northwestern foreland basin of the Himalayan fold-&-thrust belt in Pakistan. Creaming curve exhibits a bullish trend since first discovery at Chanda only a decade ago. Gas and oil occur in multiple reservoirs. Although new prospective structures continue to emerge, ambiguity surrounds the definition of plays as distinct reservoir-seal pairs and geological inter-dependencies. Practical implications are: resources under-estimated, misleading success/failure analysis of wells and ambiguous wellsite geology decisions. This is an attempt to define key play and document its sub-plays/part-plays within a sequence stratigraphic framework built through an integration of outcrop, wireline log and seismic data. Key risk elements have been assessed to draw Common Risk Segment (CRS) maps and prospectivity corridors.

The Jurassic-Eocene sedimentary succession of interest was deposited on the northwestern margin of the Indian plate in restricted to shelfal marine setting in the form of six mega-sequences. The Lower Cretaceous Chichali-Lumshiwal sequence, Upper Cretaceous Kawagarh sequence, Paleocene Hangu-Lockhart sequence, and Paleocene-Eocene Patala-Panoba-MamiKhel mega-sequence contain the reservoirs and seals of the key play of interest, the Lumshiwal-Hangu-Lockhart play. Reservoirs are charged through juxtaposition against prolific upper Paleocene Patala marine shale source. Regionally extensive thick Panoba-Mamikhel shales and evaporites cap and laterally seal the reservoirs in highly tectonized structural culminations to make the play work. The Lumshiwal reservoir is fluvio-deltaic to strandplain shoreface sandstone and correlates with the Lower Goru, a prolific play in the south. Stacked Lumshiwal-Hangu sandstones with dual porosity (matrix-fracture) provide key storage space. Overlying tight but fractured Lockhart Limestone serves to drain the hydrocarbons.

Regional correlation and Wheeler diagram reveal a Shelf Margin Systems Tract in the form of calcareous shale and marly limestone deposited further basin-wards from the Lumshiwal coastal plains. Consequently, three stacked reservoir formations are split in the northwest to provide two sub-plays, Lumshiwal and Hangu-Lockhart, as also confirmed by the discoveries like Shekhan-1.Therefore, independent resource assessment of the two is essential. Wellsite decisions to drill deeper or abandon should also be made accordingly.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.