Integrating Geologic Concepts and Geophysical Analyses to Unravel Hydrocarbon Prospectivity of Post Carbonate Shallow Clastic Plays; SW Luconia Province, Sarawak
Authors: Tukimin Nurmaliza , Abd Aziz Annas , Nayak Satyabrata , Ali M Firdaus , Zainun Fakhitah , Brewer Martin Stephen 
Published: May 2016

This paper discusses the hydrocarbon prospectivity in the post carbonate shallow clastic reservoir in the SW Luconia province, offshore Sarawak, in viewof a recent gas discovery in 2013. The well X1, situated in a water depth of 60m, was drilled to a total depth of 2900mTVDSS targeting both pre and post carbonate plays. A total net gas of 75m was encountered in both plays. Gas discovery in the shallow reservoirs of Cycle V & VI is significant as not many wells in the region have tested this emerging play. Gamma ray log motifs indicate presence of channel sands (7–20 m thick) deposited in a possible shallow marine shore-face environment. The thicknesses of the sands are similar to the reservoirs encountered in the vicinity. These sands are found to be associated with stratigraphic closure with a very subtle structural component to it. The gas bearing reservoirs (at 700 m to 900 m TVDSS) correspond to anomalous high amplitudes on the seismic sections characterised by a soft top and also show positive AVO response. Fluid substitution modelling was done to understand the seismic response to variation in fluid type (gas, oil and brine), thickness and gas saturation. The analysis enables the authors to characterise the amplitude and use the attributes in a cautious manner. Various attribute analyses like sweetness, A+B and A*B generated for the sands show presence of possible high upside potential. With the depth of less than one Km (from mud line to target), chasing these amplitude targets could be highly rewarding. The major challenges associated with shallow clastic exploration are to find out thicker reservoir, understanding amplitude response and predicting oil migration in the region. Integration of a robust geological model, well information and Geophysical data could be instrumental in pursuing shallow clastic exploration in the area.

Introduction

Offshore Sarawak Malaysia is a prospective area for hydrocarbon exploration. Previous operators were mainly targeting Carbonates and pre-Carbonates clastics. In 2013 PCSB drilled well X1. This is was an exploration well to test the hydrocarbon potential in the post-Carbonate Cycle V-VI shallow marine clastics, Cycle IV carbonates and pre-Carbonate Cycle I-III clastics. It was a deviated well drilled to penetrate all the objectives in an optimum location. Out of the seven objectives, five were found to be gas bearing and two objectives were water bearing, namely Intra Cycle IV carbonates and Cycle II clastics. Hydrocarbon discovery in the pre-Carbonate reservoirs is not new to the region, but the gas bearing sands in the post carbonate clastic reservoirs were the highlight of the well. The shallow objectives, Cycle VI, Cycle V & Intra Cycle V, are DHI amplitude driven reservoirs and all three are proven to be gas bearing supported by LFA and fluid samples. The gas discovery in the post carbonates is significant for future exploration plays. Along with the oil discovery in well ‘XX1’ (near to the study area) within post carbonates level, this emerging play is worth to be looked into by integrating geological and geophysical knowledge. Hence, this paper aims to discuss this significant discovery, the challenges faced in finding prospective shallow clastics play and the geological and geophysical workflow involved to support shallow clastics prospects.