Three Dimensional Geometry of Passive-roof Duplex, Quaternary Transpression, and Hydrocarbon Traps in the Sulaiman Foreland, Pakistan

Abstract

LANDSA T & SPOT scenes combined with geological and geophysical data have been used to examine the structural style, timing of foreland deformation, and hydrocarbon trap forming geometries in the foreland of the Sulaiman fold belt of Pakistan. Seismic reflection interpretation suggests a 10 km depth for the top of the crystalline basement atthe deformation front in the eastern and southern, and about 14 km in the western (Sibi trough) Sulaiman fold belt. Nearly all of the sedimentary strata, detached from the basement with floor thrust in Paleozoic strata, is uplifted several kilometers higher than its regional stratigraphic level without an exposed thrust in the foreland. Surface e: I ression of the deformation front is of a monocline whose steeper limb dips toward the foredeep. Overall structural style is interpreted as of a passive-roof duplex with a roofthrust in Cretaceous shale (except frontal part of western Sulaiman where it is in Eocene shale). The duplexes are exposed only in the western Sulaiman fold belt due to thickening of duplex package, and friction related to drag at the lateral termination of the fold belt.
The passive-roof sequence extends over several duplexes in the foreland and shows out-of-sequence faults in the more internal parts of the Sulaiman fold belt. According to seismic reflection data,these are reverse faults with minor vertical offsets of about 1-3.5 km and are mostly restricted to the roof sequence. Some faults show dominant strike-slip displacement (about 4-5 km). Along these faults passive-roof duplex geometry is being evolved to form backthrusts and modified by strike-slip displacement in the central Sulaiman fold belt. Persistent dominant dextral displacement along strike-slip faults suggests anticlockwise rotation of discrete blocks in theSulaiman fold belt due to its proximity to the western sinistral boundary ofthe Indian plate. Active nature offaults, based on seismicity and cross-section balancing, 10epartment of Earth Sciences, Quaid-i’-Azam University, Islamabad, Pakistan. suggests onset of Quaternary transpression in the Sulaiman fold belt.Sequential evolution of structures can be interpreted as: 1) low amplitude, long- wavelength detachment folds at the tip of the decollement; 2) passive-roof duplex geometry; 3) out-of sequence faulting; and 4) onset of Quaternary transpression.
Several structural geometries form important hydrocarbon traps. Some examples include the system of fault tip-line detachment folds (Sui and Loti/Zin gas fields), fault- propagation folds (Uch gas field), fault-bend folds (Pirkoh gas field), several duplex geometries (unexplored), and secondary traps in the footwall.