| Abstract |
Integrated geology, geochemistry and geophysics data shows numerous reservoir characteristics at Darajat that are similar to other known vapor geothermal systems. The Darajat surface manifestations are characterized by the appearance of fumaroles, mud pools and steam condensate fed springs only, with the absence of the typical sodium chloride hot springs. Analysis of thin sections and temperature dependent alteration minerals showed that the vapor field first started as a water dominated system with reservoir temperature exceeding 280 Ô C and has cooled to the current 240 Ô C vapor reservoir. Intrusive rocks, temperature dependent alteration minerals, gas ratios, and top of reservoir contours suggested that the magma bodies that supply heat to the previous liquid and the current vapor systems are apparently centered in the same vicinity. MEQ data and seismic tomography modeling suggested that the younger intrusive bodies that supply heat to the vapor system must be situated deeper than 4 km from the current surface. Argillic (illite-smectite) and silicic alteration product deposited near the top of the previous liquid reservoir, capped and sealed the current vapors reservoir. The increasing gas/steam, CO2/H2S and CO2/NH3 ratios observed with commercial steam production support the idea of liquid water in the rock matrix (boiling). Gas ratio contours overlap with the current top of the reservoir identify the up flow and out flow zones. No convincing geochemistry data indicate any natural recharge from the reservoir margins (sides) or input from deep chloride brines. Cores, cuttings and thin sections work showed that alteration clays and minerals deposited by the up flowing fluids from the past liquid system have a significant control on the plumbing and fluid path of the current vapor reservoir. Numerous veins, voids and vugs in the rock matrix are filled and sealed with alteration minerals. Formation Micro Scanner data suggest that major permeable zones in the vapor reservoir may be characterized by young, unmineralized, high density minor fractures or fractures where the deposited minerals have been leached by downward flowing steam condensate. |