CONTROLS ON FACIES DISTRIBUTION AND PROPERTIES IN THE NORTHWEST BORNEO DEEP WATER FOLD-AND-THRUST BELT

Abstract

The deep-water fold-and-thrust belt of NW Borneo contains a structurally complex region of active folds. The opening of the South China Sea during Oligocene to Middle Miocene times led to a Southeastward rifting of the continental crust of the Dangerous Grounds. Subduction of oceanic crust to the Southeast took place under the NW Borneo continental margin, with oceanic subduction ending at ~16 Ma. Collision between the Dangerous Grounds and the NW Borneo margin resulted in uplift and erosion of the Crocker Ranges, and shed large volumes of sediment Northwest into the adjacent sedimentary basins. The resultant fold-and-thrust belt hosts up to 8 km thickness of Mid-Miocene to Recent turbidites and submarine fan sediments, closely associated with fold growth. This study provides new interpretations of regional, three-dimensional (3-D), multichannel seismic-reflection profiles, and utilises data from 25 exploration and appraisal wells. The target of the wells is the fan system, located at the crestal part of the thrusted hanging walls of growth anticlines. This research studies the geometry and structural vergence of the folds, sequence stratigraphy, and burial history. Collectively, these are the controlling factors affecting the facies distribution and properties in the study area. Approaches include seismic analysis, well log analysis, petrology, and basin modelling. Fold-and-thrust belt evolution and syn-tectonic deposition were influenced by subduction and gravitational tectonic events, the proximity and variety of sediment provenance, and seabed topography during deposition. There was an important effect of overpressure generated by disequilibrium compaction, which preserved the porosity in the deeper formations. This study identified the individual fold growth magnitude, sediment provenances, depositional settings of the deep-water slope water environment, and accommodation space for the facies compartmentalisation seen in the wells, and models the depositional settings and the submarine channel pathways, with possible high quality reservoir distributions.