Abstract

Approximately 100 years after the first oil discovery at Langkat in northern Sumatra, geologists from the British Geological Survey and LEMIGAS began a collaborative study to reassess the geology and hydrocarbon potential of the onshore central North Sumatra Basin. Results indicate the possibility of undiscovered hydrocarbon accumulations in stratigraphic traps and structural closures within the Miocene sediments, and within Palaeogene half-grabens which are believed to have been the source kitchens.

Reappraisal of basin stratigraphy initially involved recognition and mapping of seismic sequences. Subsequent integration of biostratigraphic and geophysical log data has allowed refinement of sequence boundaries and the identification of depositional systems tracts; redefinition of certain established rock units and an amended stratigraphic nomenclature for the Miocene succession are proposed. Reassessment of sandstone units in terms of lowstand, transgressive or highstand depositional systems tracts has assisted prediction of the distribution of potential reservoir units in the basin. In combination with sedimentological studies this stratigraphic work has revealed the existence of possible untested stratigraphic traps within the Middle to Upper Miocene succession.

Geochemical analyses show that marine mudstones in the study area have poor source potential for gas only. Gas chromatography / mass spectroscopy and stable isotope analyses of selected reservoir oils indicate the source rocks to have been largely lacustrine and very mature. These source rocks are probably located within Palaeogene half grabens which have yet to be drilled.

Basin modelling suggests oil generation began at approximately 11 Ma in the deepest of the half-grabens, with migration facilitated initially by post early Miocene faulting; subsequent migration was related to Plio-Pleistocene faulting and folding. With a deep source and fault conduits, hydrocarbon accumulations are possible throughout the Tertiary succession where suitable traps and seals are available. However, petrological studies show that reservoir quality generally deteriorates with depth in the basin.