Online Journal for E&P Geoscientists

Amthor, Joachim ^*1; Ramseyer, Karl ²
(1) PDO, Muscat, Oman. (2) Bern University, Bern, Switzerland.

The Al Shomou Silicilyte in the South Oman Salt Basin is an example of an unconventional light tight-oil (LTO) play, which Petroleum Development Oman (PDO) has successfully developed since 2000 using massive fraccing technology.

The silicilyte shares many of the characteristics of a LTO play: It is a prolific source rock mature for light oil, it produces light oil from a very tight matrix, and fraccing technology is needed to produce the oil. What is intriguing about the Al Shomou Silicilyte, and different from ‘conventional’ LTO plays, is its position at the Precambrian/Boundary and that it is a ‘laminated chert rather than a shale.

To shed light onto the source of silica and its mode of formation we applied microstructural analyses (SEM, BSE and TEM investigations) combined with fluid inclusion microthermometry, state-of-the-art stable isotope analysis and trace element analyses of the silicilyte matrix and fractures.

The microstructural investigations reveal a fine lamination of the silicilyte with a mean lamina thickness of ca. 20 µm consisting of predominantly organic matter-rich and finely crystalline quartz-rich layers, respectively. Authigenic idiomorphic quartz crystals are the main components of the silicilyte. Other clearly diagenetic phases are pyrite, apatite, dolomite, magnesite and barite. TEM analyses of silicilyte devoid of organic matter clearly indicate that quartz is micron-sized and idiomorphic where it grows into open spaces.

The observed open intercrystalline spaces are extremely varied, with irregular one's dominating. Pore casts of these spaces are extremely irregular in three dimensions.The maximum open space diameter from BSE analysis averages 1.5 µm with an average throat diameter of 0.4 µm. This is in line with mercury capillary pressure data, which indicate a pore-throat modal diameter of 0.4 microns.

The newly acquired geochemical data support our interpretation that the primary source of the silica is the ambient seawater rather than hydrothermal or biogenic sources, e.g., sponges. The Al Shomou Silicilyte appears to have formed largely by direct silica precipitation from a silica gel at shallow depth below the seabed. A maximum temperature of ~70°C for the formation of microcrystalline quartz in the silicilyte is good evidence that the lithification and crystallization of quartz occurred in the first 50 Ma after deposition. Fluid inclusion data indicate the presence of hydrocarbons in the silicilyte already at this stage.