--> Deep Clastic Reservoir Quality in Ciputat Low, Onshore North West Java Basin

AAPG Asia Pacific Technical Symposium

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Deep Clastic Reservoir Quality in Ciputat Low, Onshore North West Java Basin

Abstract

A regional porosity vs depth model for sandstone has been developed for onshore North West Java Basin using ambient porosity from routine core analysis data and sonic porosity. The area from west to east covers Ciputat, Kepuh, Cipunegara, Cemara, Waled, Jatibarang and Babadan Low. By assuming there is no significant erosion at the surface as indicated from vitrinite reflectance, seismic and sonic value, the porosity curve follows exponential trend with surface porosity 58% and slope coefficient -6.3*10-4 as best fit model. The model also tells us that below depth 3000 mTVDss the porosity will be less than 10%, with permeability <0.2 mD. Low estimate model predicts porosity 5% higher in average and high estimate model predicts porosity 6% lower in average than the best fit model. Attempt has been made to characterize the low and high estimate porosity model using petrography classification and led into bias conclusion due to limited data. We found an appropriate technique to characterize the porosity model using GR log pattern scale 1:200 ; cylindrical, funnel, bell, symmetrical and serrated. Cylindrical log pattern follows low estimate model better and may reflect high energy play. The basin wide porosity model then tested in Ciputat Low following company strategy to drill deep wells (more than 3000 meters) in the area. Our last two wells in the area targeted sandstone of Equivalent Jatibarang Formation as primary objective, and found both reservoir and hydrocarbon but failed to flow hydrocarbon in significant quantity due to reservoir quality problem. The porosity model within Ciputat Low agrees with the basin wide porosity model. Fortunately, large petrography data are available within this Ciputat area and ease us to characterize the porosity model. The scheme used for the analysis is the combination between Folk (1974) classification and Dott (1964) classification. Best estimate model deals with arkosic sandstone, while high estimate model deals with wacke sandstone and low estimate model deals with arenite group sandstone (arenite, sublitharenite and litharenite sandstone). The arkosic sandstone and wacke sandstone are found in relative deep area and interpreted as sand bars deposited in estuary setting as indicated from core, log, and seismic attribute. Arenite group sandstone mostly found in high area ; Rancajawa, Pondok Tengah and reflects channel deposition as also indicated from core, log, and seismic attribute. The comprehensive analysis of compaction for different rock clusters suggests that the main factor of porosity reducing in the study area is grain composition. Wacke sandstone undergo bigger degree of compaction than other groups due to the presence of more clay minerals. Diagenetic play minor role in porosity reducing for arenite group sandstone as indicated from petrography with quartz overgrowths only counts maximum 5%. As generally accepted, diagenetic plays important role in decreasing permeability for sandstone, especially by the presence of some authigenic clay minerals. We identified these kind of minerals from thin section such as ; kaolinite, illite and chlorite, but no clear relationship observed between permeability and clay content due to very limited XRD data. We believe that the very low permeability value for arkosic sandstone and wacke sandstone is caused by very low porosity.