Geochemical Characteristics of Gas Microseepage from Mimika, Papua, Indonesia
Asep Rohiman*, and Edy Slameto
Center for Geological Survey, Geological Agency, Ministry of Energy and Mineral Resources. Republic of Indonesia
Jl. Diponegoro No. 57 Bandung 40122, Indonesia
*The corresponding author’s e-mail address : asep.rohiman[at]esdm.go.id
Abstract
Fossil fuels are the main fuel that is still used by the community. Along with the increasing population of Indonesia, the need for oil and gas fuel is also increasing. Therefore, oil and gas exploration activities are needed to obtain new economical reserves. A regional surface soil gas survey consisting of 300 sites were placed on approximately 1 km intervals using existing rivers, streams and navigable waterways within the onshore portion of the Boka area located in Mimika Regency in Papua for evaluation of the hydrocarbon potential. This soil gas data contains very large magnitude anomalies consisting of a unique mix of light hydrocarbons (methane, ethane and propane) and carbon dioxide. Generally very large concentrations of methane and CO2. In this case, these two gases (methane and CO2) have a clear association with the larger petrogenic ethane and propane gases. In addition these coherent anomalies occur at sites where the ethane/propane ratio exhibits somewhat noisy ratios with some sites having ratios less than one, a condition that is generally found only within macro gas and oil seepage environments where both volatilization and degradation can randomly alter their ratios. The logical conclusion is these biogenic gas anomalies associated with noisy ethane/propane ratios are evidence of a macro seepage environment. In spite of the very regional spacing (1 km) the largest magnitude anomalies clearly suggest two main clusters of anomalies. One occurs on and along strike with the Aiduna fault zone, and the second clusters along the coast line near the mouth of the Aiduna and Umari rivers. These anomalies suggest deep source oil potential exists over the entire area surveyed. The Aiduna River traverse crosses the northern trace of the inferred Aiduna fault with more impressive anomalies, including the largest magnitude propane and C2+ gases found, following fairly close to the Aiduna fault trace. These oily soil gas anomalies confirm the existence of a wide-spread and active subsurface petroleum system. Given adequate density, it appears that future soil gas surveys could provide a cost effective approach to focusing on and defining areas which could be further developed before much more expensive exploration tools (seismic) and drilling are employed. An offshore sampling survey might confirm and extend these coastal anomalies into the adjacent offshore.
Keywords: microseepage, geochemistry, mimika, oil and gas, gas chromatography
Topic: Earth and Planetary Sciences