The Matalibong-25 Corehole at the TIWI Geothermal Field

Abstract

Well Matalibong-25 was core-drilled to 2439 m (80009 in the Tiwi reservoir as a pressure monitoring well in 1992-1993. While core drilling overcame the technical challenge of drilling a deep well into an underpressured reservoir, the scient@ results of the recovered rock core have added significantly to the value of the well. Analysis of the core has yielded detailed data on stratigraphy and hydrothermal alteration,, and characterized reservoir permeability and the nature of the reservoir floor. Well test results show that cored slim holes can quickly give stabilized formation temperatures, and when flowed, yield well test parameters and fluid samples comparable to that of a regular-sized production hole.

US DOE-sponsored research on the rock core has provided important insights into the chemise and evolution of the Tiwi hydrothermal system. Studies of fractures and vein occurrence and orientation show that reservoir permeability is due to secondary faulting and fracturing, without little or no influence of primary lithology. Detailed studies of veins show that the system underwent eight distinct stages of mineral precipitation, culminating in an assemblage of illite-quartz. Geochemical modeling of the present day reservoir fluids shows that they are in equilibrium with this last illite-quartz assemblage, and appear to represent a mixture between fresh and seawater. Fluid inclusion gases, analyzed by quadrupole mass spectrometry, reveal a meteoric character to the early system and higher concentrations of methane than in the present system. '%r/agAr age dating of vein adularia suggests an age of >200 ka for the frst 7 stages of veining, and a recent rejuvenation of the system after an extended period of quiescence. Ongoing and future research includes additional age dating, noble gas isotopes of fluid inclusion gases and studies into the nature of porosity in the core.