CHARACTERS OF SEDIMENTOLOG Y, ROCK PROPERTY AND GEOCHEMISTRY OF THE NGRAYONG AND TUBAN FORMATIONS IN THE PATI TROUGH, ONSHORE NORTH EAST JAVA BASIN

摘要

Exploration activities in the Pati Trough, onshore of North East Java Basin in the northern part where it is also situated in the Rembang Zone of the great Rembang Madura Kangean Sakala (RMKS) Fault Zone, recently successfully found gas and condensate derived from Early Miocene Tuban and Middle Miocene Ngrayong Formations. Further investigation, based on outcrop and well data set, is conducted to understand sedimentology, rock property and geochemistry characters and its implication to the petroleum system of reservoir and source rock. Sedimentological analyses indicates that the Tuban Formation is composed of marine shale, boundstone, larger foram limestone and calcarenite, channel and sandbar lithofacieses, whilst the Ngrayong Formation is composed of marine shale, tidal shale, tidal sand, larger foram limestone, channel lithofacieses and the last is sandbar lithofacies as dominated facies. Early to Middle Miocene regional paleoshoreline were situated tens of kilometers to the northwest of the present day Bawean Island. However, the Ngrayong and Tuban Formations in the Pati Trough were deposited in transitional to shallow marine environments where coal layers, coaly shales and gypsum are observed in Ngrayong outcrops. Hence, the depositional environment in the Pati Trough should be controlled by inversion tectonic relating to Neogene north-south regional compression of the RMKS Fault Zone that commenced since Early Miocene. Rock geochemistry character of the Ngrayong and upper part of Tuban Formation indicate immature source rock but has relatively high Total Organic Carbon. Further confirmation by gas isotope analysis from proven upper part of Tuban and Ngrayong reservoirs indicates biogenic contribution source to the reservoir. Cross plotting porosity and permeability against age indicates three linear trends of good-fair-poor, in which it can be further used to estimate flow connectivity within a single reservoir. A good trend is strongly related to possible connectivity, a fair trend might be related to probable connectivity and a poor trend most likely means unconnected. Ngrayong and Tuban sandstones share similar characters of grain size, shape, contact and quartz to ductile grains ratio suggest a similar provenance, transportation and overburden effect. Nevertheless, the Tuban reservoir has poorer property in P#l exploration well due to telogenesis process where exposing limestone become a source for calcite solution dripping downward through fractures to fill porosity, so that greatly reduce rock properties.