Compaction history based on porosity, pressure and temperature relation in Central Luconia province, offshore Sarawak

Abstract

This study investigates the role of porosity in relation to temperature and pressure within Miocene-Pliocene sediments in Central Luconia province. The main objective is to understand the effect on porosity loss within Miocene-Pliocene sediments as part of the compaction of the sediments through time. The methodology involves seismic interpretation of 2D seismic lines and analysis of four wells over the central complex in Central Luconia province. Thick sedimentation was found within the clastic sediments of Cycle VI, indicating major clastic influx from the deltas and onshore of Sarawak during Pliocene. These clastic sediments had become a major source of weight in the offshore basin, inducing compaction within the older formations. Porosity distributions based on the wells showed that Cycle VI had the highest porosity in LH1 well with 33.77% porosity, while porosity within Cycle IV & V limestone showed an average porosity of 22% in all wells. Porosities within Cycle III and Cycle II were observed to be average of 16%. The porosity within the reservoir rock was reduced as the depth increased. This porosity reduction was relatively due to the compaction of the sediments, linked with the changes of pressure and temperature indicated by several evidences found in each well. Overpressure zone affects irregularity of porosity reduction with depth, whereas high temperature promotes diagenetic reaction that causes sudden porosity loss. Nevertheless, a sudden increase in porosity was also observed at the top of Cycle IV & V limestone due to a major drop in the sea level during the Early Late Miocene, which had encouraged the karstification process, inducing secondary porosity. This study has proved that compaction within Miocene-Pliocene sediments is not only related to the loss in porosity, but it involves changes in pressure and temperature regime of sediments and is highly dependent on the natural changes of sea level.