DEPOSITIONAL ENVIRONMENTS AND RESERVOIR ASSESSMENT OF LATE CRETACEOUS SANDSTONES IN THE SOUTH CENTRAL KIRTHAR FOLDBELT, PAKISTAN
A. S. Khan1 G. Kelling2*, M. Umar1 and A. M. Kassi1
2School of Earth Sciences and Geography, Keele University, Staffordshire,ST5 5BG.
1Department of Geology, University of Balochistan, Quetta, Pakistan.
* Author for correspondence, email@example.com
Sandstones of Late Cretaceous (Campanian-Maastrichtian) age constitute much of the Pab and Mughal Kot Formations which are well exposed in western Pakistan. In the south central part of the Kirthar foldbelt, these units attain thicknesses ranging up to 350m and are dominated by quartzose sandstones with subordinate argillites and marls, all deposited on the western continental margin of the Indo-Pakistan Plate. Seven sedimentologic logs, measured through this sandy sequence along a strike-wise extent of some 290 km, provide new data on the palaeoenvironments and dispersal patterns represented in these rocks and on their reservoir potential.
The eight main facies were formed by a range of sand-transporting mechanisms that includes classical turbidity flows, storm- and river flood-generated underflows, high and low-energy tractional flows, storm- and fair weather wave reworking, and local slope failure and remobilisation. The four facies associations comprise a Shoreface Association, a Shelfal Delta Lobe Association, a Deeper Shelf/ Ramp Association (the distal equivalent of the Shelfal Delta Lobe Association); and a deep-water Submarine Fan Lobe Association.
Facies distributions and palaeoflow patterns demonstrate the existence of two depositional systems in this area during the Late Cretaceous. Sandbodies formed in a NW-prograding submarine fan dominate a southern system, while in the north of the study area, storm- and river flood-influenced shelfal sands were conveyed to the west and WNW. Here, sediment-gravity flows evolved from east to west, following a broadly across-shelf pattern from cross-bedded sandstones to graded turbidites. Sandstone petrography suggests supply from the uplifting Indo-Pakistan shield to the east, feeding a broad, west-facing shelf or clastic ramp characterised by Mutti-type "shelf-lobes".
At outcrop, these sandstones possess low porosities but other reservoir characteristics (total sand thickness, percent sand, sand-body geometry, vertical and lateral connectivity, etc.) are closely linked to dominant facies types and their inferred paleoenvironments. Using these attributes, sandstones formed in the western, mid-shelf, settings of the northern system display the most promising reservoir properties.