TECTONO-STRATIGRAPHY OF BLOCK A AREA, NORTH SUMATRA BASIN: THE IMPACT OF LOCAL TECTONICS AND EUSTASY TO ACCOMODATION SPACE OF THE TERTIARY INTERVAL

摘要

North Sumatra Basin (NSB) is located in tectonically active area within the margin of Sibumasu continent, SE Asia, bounded by the great Sumatran Fault to the south and southwest and the Andaman Sea rifting to the northwest. Strong evidences of active tectonics controlling the basin development and basin fillings are widely recognized from its regional structural patterns and sedimentary sequences. Despite the quite established subdivision of NSB tectonic phases from various researchers (rift, post-rift and syn-tectonic or inversion), some discrepancies remain, pertinent on the timing of tectonic events, age of sedimentary sequences and basin definition. In addition to that, the effect of tectonic activities that are reflected by local uplift and subsidence varies across the basin, promoting a different control on the accommodation space and sedimentation rate dictated by its relative position within the basin (basement high and low). Regional evaluation was conducted to revisit the tectono-stratigraphy of NSB. Chronostratigraphic charts and Gross Depositional Environment (GDE) maps of each sequence were constructed to re-define and illustrate correlative sedimentary sequences and depositional facies distribution in both vertical and lateral perspective. Integration with regional geology-is helpful to identify areas where potential uplift and subsidence occurred within the basin. This paper highlights the effect of local tectonic within Block A area and its impact to the accommodation space through time in comparison to the effect of eustatic variation. Generated subsidence curve from selected deep wells demonstrates the predominant control of local tectonics compared to the regional eustasy during Late Oligocene to Plio-Pleistocene period. The study results highlight the major subsidence occurred during the Paleogene rifting and Middle Miocene to recent syn-tectonic episodes, separated by a period of tectonic quiescence in Early-Middle Miocene (6.5Mya). These local tectonics play an important role in controlling the petroleum system components in the basin particularly in the reservoir distribution and quality, top-seal development, source rock distribution and maturation.