Fine-grained deep-water turbidites, Baram Delta Province, Offshore Brunei Darussalam

Abstract

Thin-bedded turbidites dominate the deep-water succession of the Mio-Pliocene to Recent offshore Brunei Darussalam. This reflects the physical conditions conducive for the deposition of low-density turbidites by low energy sediment gravity flows. The focus of this study is to develop an understanding of these thin-bedded turbidites based on an integrated analysis utilising subsurface 3D seismic, well data, and surface outcrops through the application of both qualitative and quantitative approach. This study was conducted on the Miocene-Pliocene Baram Delta Province (BDP), focussing on the eastern slope of deep-water Brunei. From seismic data, the emphasis is on the origin of the high amplitude events deposited in the minibasins along the slopes. Quantitative interpretation from well data indicate (i) stacks of high amplitude events are interpreted to be caused by variations in lithology with relatively acoustically hard sand being modified by diagenetic process, encased by soft shale, and (ii) the apparent dominant thin-bedded turbidites deposited on the Brunei slopes reflects the efficient trapping of coarser sediments on the shelf behind the counter-regional growth fault. The sedimentology and environment of deposition of these thin-bedded turbidites are studied using a nearby Miocene outcrop succession. Detail analyses suggest the outcrop are: (a) low-density turbidites deposited by sustained, but overall decelerating flows with unidirectional palaeocurrent consistent with the palaeoslope of the Early Miocene, and (b) vertical stacking of sandstone beds reflect aggradational stacking of lobe elements in a lateral off-axis/fringe position. Quantification of bed thickness trends and vertical heterogeneity show: (1) outcrop data indicate statistically significant of thickening-upward (TkU), (2) limited applicability on the use of Hurst statistics to interpret depositional sub-environment, and (3) heterogeneity is governed by sampling protocol where increase in sampling frequency and decrease in sampling resolution, decreases heterogeneity.