Publications

Unconventional reservoirs rely heavily on analog data to define sweet spots early in the exploration history of a basin. The Indonesian unconventional marine prospects such as the Talang Akar, Jatibarang and Bampo Shales (Sumatra and Java) were deposited in a back arc setting. This is a very different tectonic and depositional setting than most successful unconventional analogs in North America, deposited in plate-interior basins, which are typically used as analogs. A better comparison is made with Vaca Muerta (Neuquén Basin, Argentina), an analog with a similar tectonic and depositional history as the Tertiary Indonesian unconventional prospects. The Vaca Muerta was deposited between an active, semi-continuous volcanic arc and stable continental crust (Fig. 1). It was deposited as a mixed carbonate-siliciclastic ramp (shelf) with sediment coming from the continent and volcanic arc, and “in-situ” production of shallow-water carbonates and planktonic organisms (radiolarian, coccoliths, algal fragments, foraminifera,..). The Vaca Muerta is Late Jurassic in age (Fig. 2), occupies an area of at least 60,000 km2, the thickness ranges between 100-500 m, and internally exhibits clinothem geometries arranged in aggradational, progradational and degradational packages (Fig. 3). The sediment mineralogy indicates near equal parts of silica, carbonate and clay. The mineralogy is in partially depositional, but diagenesis in the form of calcite replacement of silica, calcite and silica cement authigenesis, and albite crystal growth impact the reservoir quality and brittleness. Discrete layers of volcanic ash disseminated throughout the stratigraphy also have a major impact in brittleness. The Vaca Muerta TOC varies between 3-7 %, and has a porosity range of 7-17%, low permeability (20-600 nD), and * ExxonMobil Exploration Company, Houston ** ExxonMobil Upstream Research, Houston ranges in maturity (Ro) between 0.6 and 2.0+. The “best” reservoir, the rocks that contains storage of and is able to deliver (artificially or naturally) hydrocarbons, appears to be the intervals with the thickest medium-grained-silica-rich mudstones that occur in the middle of parasequences. These units have the best preserved coccolith-pellets, are partially enriched in organic carbon and relatively depleted in clay minerals and are partially enriched in silica and carbonate which maintains the pore space. The coarsest units at the tops of parasequences have had their reservoir qualities degraded because either (a) microbial-mediated cementation associated with a pause in sedimentation during flooding has filled the precompaction porosity or (b) because original siliceous radiolarian tests have been replaced by burial reducing the volume of porosity present. Conversely, the beds at the base of parasequences have had poorer unconventional reservoir quality because they have been bioturbated, have few coccolith-rich fecal pellets and have the highest clay mineral-contents. These ranges in value that define the “best” reservoir and brittleness may be similar to the Indonesian unconventional prospects.