The Significance of Chemical Compaction in Modeling the Overpressure in the Malay Basin

M. Jamaal Hoesni¹, Richard E. Swarbrick², and Neil R. Goulty^3
1 PETRONAS Group Research, Lot 3288 & 3289, Kaw. Institusi Bangi, Off Jalan Ayer Itam, 43000 Kajang, Selangor, Malaysia
² Geopressure Technology Ltd, Mountjoy Research Centre ,Stockton Road, Durham, DH1 3UZ,United Kingdom
³ Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE,United Kingdom

The Malay Basin is an overpressured Tertiary rift basin located on the Sunda Shelf, bounded by the Pattani Basin to the north and West Natuna Basin to the south. The sedimentary history of the Malay Basin began in the Late Eocene-Early Oligocene (45 Ma), at the same time as the Indian and Eurasian Plates collided, resulting in the creation of mountain belts. The mountain-building process that resulted from the Indian-Eurasian Plate collision allowed enormous quantities of sediments to be shed into the surrounding basins. High sedimentation rates are recorded in the Malay Basin and have been considered as the main contributory for the development of overpressure in the basin.

Onset of overpressure is observed to be shallower in the basin center (about 1400 m) and deeper towards the basin flanks (about 3000 m). High excess pressure recorded in the Malay Basin can be reconstructed through rapid deposition of fine grained sediments (low permeability on the order of 1nD). The observed low porosity of the shales relative to effective stress is not explained by disequilibrium compaction, but rather by associated chemical compaction related to a high geothermal gradient in the area. We have attempted to model this phenomenon.

A series of 2D basin modeling analysis were performed, to understand the pressure histories resulting from burial (stress), hydrocarbon generation and changes in rock properties due to chemical compaction processes. Modeling can successfully replicate pressure profiles when appropriate porosity-permeability histories are considered, taking into account the effect of chemical compaction. The creation of effective pressure seals through chemical compaction helps to sustain higher excess pressure in the basin. A fixed porosity-permeability relationship for shales is not suitable for modeling fluid-flow and overpressure in areas where shales are known to be affected by chemical compaction. A fixed porosity-permeability relationship creates a large undercompaction, leading to much higher fluid retention, sealing and geothermal gradient than observed. A temperature-controlled porosity-permeability relationship, however, takes into account the effect of chemical compaction in reduction of porosity and permeability of the shales. The occurrences of chemical compaction events are corroborated by the wireline log signatures. The reduction in permeability due to chemical compaction has been shown to have important implications in some parts of the basin and attests to the need for a much more sophisticated porosity-permeability relationship in basin modeling, where the influence of time-temperature is included.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands