EVALUATION OF THE MESOZOIC STRATIGRAPHY OF MISOOLISLANB AND IMPLICATIONS FOR PETROLEUM EXPLORATION IN THE BIRD'S HEAB REGION, WESTPAPUA, INDONESIA

摘要

Misool Island is located in Eastern Indonesia about 150 km southwest of the city of Sorong and 350 km west of the Tangguh gas fields. Field observation of Mesozoic outcrop on the island was undertaken in mid-2010 and laboratory analysis of field samples was performed to define the age, depositional environment, source rock potential and petrophysical properties of the outcrop.The Bogal Limestone is the oldest sequence observed in the field; it is a massive limestone and defined as Rhaetian (Late Triassic) age. It was deposited in a marine environment with a moderate terrestrial influence.The Bajocian (Middle Jurassic) Yefbie Formation was observed as thick black shale with limestone concretions. The shale was deposited in a marine setting with a strong terrestrial influence and is likely the distal facies of the Roabiba Sandstone Formation penetrated in Tangguh area in Bintuni Bay. The shale is underlain by thin bedded sandy limestone and a basal conglomerate. The sandy limestone is defined petrographically as recrystallized sandy limestone with quartz and bioclast grains. The sandy limestone is of Toarcian (Early Jurassic) age, equivalent to the Early Jurassic sandstones penetrated in the East Onin-IST and TBJ-IX wells. The Demu Formation was observed in outcrop as well bedded mudstone with intensive fracturing. It is of Bajocian (Middle Jurassic) to Late Jurassic age and was deposited in a marine environment with very strong terrestrial influence. The Demu Formation is overlain by the Oxfordian (Late Jurassic) Lelinta Formation shale that was deposited in marine environment with very low terrestrial influence.The youngest Mesozoic sequence observed is the Facet Formation limestone of Tithonian (Late Jurassic) to Berriasian (Early Cretaceous) age; it is a recrystallized wackestone with no visible porosity observed in thin section. The limestone was deposited in a marine setting with very low terrestrial influence and bathyal settings.The best potential source rock based on geochemical analysis is the Middle Jurassic Yefbie Formation shale with TOC up to 1.9% and HI of 120 to 180 mgS2/gT0C. The kerogen type is D/E (terrigenous) or Type III categorized as gas prone with an expellable potential up to 0.25 bcf/km^ for every meter source rock. It is very similar to the Jurassic source rock identified in the East Onin-1 ST and TBJ-IX wells.There is no potential reservoir observed in outcrop. The Toarcian (Early Jurassic) shallow marine sandstone that was penetrated in the East Onin-IST and TBJ-IX wells was identified as a thin sandy limestone in the lower part of Yefbie Formation on Misool Island. The extent of the Roabiba sandstone and the Toarcian-aged sandstones to the west of the Tangguh fields area remains questionable. The presence of Paleocene-aged sandstones of the Daram Formation to the southeast of Misool Island could potentially provide another reservoir target.