RELATIONSHIPS OF DEPOSITIONAL ENVIRONMENT AND MINERALOGY ON ROCK FRACTURABILITY (BRITTLENESS): CASE STUDY OF EOCENE NANGGULAN SHALE, KULON PROGO, YOGYAKARTA

摘要

Geomechanics is an important parameter inrndetermining a prospective shale gas interval’srnpotential. The mineralogy (silica, feldspar,rncarbonate, and clay) and organic matter are elementsrnthat affect the geomechanical properties, which laterrnlead to a fracturability number (brittleness index, BI)rnof the shale. Any shale gas exploration will benefit ifrnthe relation between the depositional process, whichrndelivers a resultant mineralogy, and the fracturabilityrnis well known. This paper integrates the result ofrnlithofacies and depositional environment analysisrnwith mineralogical effect on the rock fracturability ofrnshale and other fine-grained sedimentary rocks.rnSamples were taken from cores representing thernEocene section of the Nanggulan Formation in KulonrnProgo, Yogyakarta. Evaluated sections wererndeposited as in estuary to shallow marinernenvironments. The estuary environment is dividedrninto the Upper Flow Regime (UFR) Sand Flat andrnTidal Flat. The UFR Sand Flat environment consistsrnof flaser-wavy sandstone domination andrninterbedded lenticular mudstone. The Tidal Flatrnenvironment is composed of mudstone facies in thernlower part and sandstone facies in the upper part.rnAbove these,, the depositional environment changesrnto shallow marine with the domination of massivernmudstone facies, sandstone facies, and increasingrncarbonate facies. Shale samples were taken from thernmudstone facies. X-Ray Diffraction data show thatrnthe variability of the mineralogical composition ofrnestuary shale is higher than the shallow marine shale.rnWith similar TOC numbers, the BI of estuary shalernwill be varied compared with shallow marine shale.rnHowever, shallow marine shales tend to have arnhigher fracturability.