Understanding and Defining the Metasedimentary Basement in Malay Basin, Offshore Peninsular Malaysia

Pendkar, Narender; Mokhtar, Puteri Nurlina Bt; Juni, Zainal Abidin B.; Kadir, Mohd B.
PETRONAS Carigali Sdn. Bhd., Tower 2, Petronas Twin Towers, KLCC, Kuala Lumpur, Malaysia

The current focused exploration for basement plays in Malay Basin has assumed great importance with the discovery of hydrocarbons from several wells in offshore Peninsular Malaysia. The understanding of the drilled sequence of metasedimentary rocks has greatly benefitted from the analogy with the outcropped rocks from the East Coast Peninsular Malaysia of Upper Paleozoic age. These metasedimentary rocks were derived from a protolith of sedimentary rocks deposited in shallow marine and coastal plain environments.

The outcropped rocks in Terengganu, Peninsular Malaysia have been intruded by granitic emplacements and to a lesser degree by mafic bodies. The rocks have been subjected to a variable metamorphic grade during different tectonic episodes, in addition to localized contact metamorphism by intrusions. This paper examines the role of various observed geological elements gleaned from outcrops for delineating effective fractured reservoir in the drilled basement.

The drilled metasedimentary rocks are composed predominantly of low grade argillites, phyllites and quartzitic rocks. The cored sections show multiple deformation histories with fracturing and mineral fill, mostly by quartz, similar to those observed on the outcrops. The deformation and the structural features along with the different fractures can be observed on FMI logs. The fractures are of several generations- the older fracture sets are mostly healed and filled with silica, making them ineffective reservoirs. However, the younger (post Middle Miocene) open fractures related to the ongoing regional tectonic stresses are critical to the development of effective fractured reservoirs. Outcrop and subsurface data on fracture orientation have paved a way for effective drilling campaign.

Systematic examination of the cores and extensive sidewall core samples by petrography has shown the presence of a number of volcanic/intrusive rock intervals of both felsic and mafic compositions along with several horizons of pyroclastics and volcanoclastics. The analysis shows that the sedimentary protolith was a complex assemblage derived from several episodes of depositional cycles and volcanic activity reflecting the tectonic evolution. The geochronological data from cores shows two episodes of metamorphic imprints. The association of hydrocarbon presence with mostly brittle quartzite and volcanic igneous rocks is important for understanding the presence of fractured zones within the predominantly ductile basement.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012