AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Hydrodynamic Fractionation of Minerals and Textures in Submarine Fans: Implications for the Prediction of Reservoir Quality
(1) Chevron Center of Research Excellence, Colorado School of Mines, Denver, CO.
(2) Clastic Stratigraphy R&D, Chevron ETC, San Ramon, CA.
(3) Clastic Stratigraphy R&D, Chevron ETC, Houston, CA.
Submarine fans are excellent oil and gas reservoirs. Outcrop and subsurface studies document that these systems consist of channels and lobes that compensationally stack. Additionally, studies show that individual lobes have a thick axis and strata thin and become finer-grained toward the lateral and distal margins. No studies have documented how reservoir quality changes laterally and longitudinally in these deposits. This study uses exceptionally well-exposed outcrops of lobe strata in the Point Loma Formation, San Diego, California to test how turbidity currents hydrodynamically fractionate mineral grains of different size, shape, and density, and the how this affects reservoir quality.
A single bed within a larger lobe element is studied over 2.5 km from axis to margin. Macro- and micro-scale parameters are documented along the profile and include bed thickness, primary and secondary sedimentary structures, mineral composition, sorting, grain size, and grain shape. There is a systematic axis-to-margin change in the character of the bed. In the axial position of the lobe where the bed is the thickest, it consists of 4 m of structureless sandstone, is well sorted, and ranges from coarse sand to pebble-sized shale clasts. The bed maintains a similar character for approximately 1.5 km. In the off-axis location, bed thickness decreases abruptly to 0.5 m, and consists of structureless sand, layers of shale clasts and silty laminae, sorting decreases, and ranges from silt to pebble-sized shale clasts. At the margin, the bed is only 20 cm thick, and consists of ripple laminated siltstone to very-fine sandstone, has poor sorting, and ranges from clay to fine sand. The bed continues laterally as a terminal siltstone. Preliminary data show that the percentage of K-feldspar increases relative to quartz along the profile. Therefore, hydrodynamic fractionation of minerals and textures is an important process that operates in submarine fans.
Hydrodynamic fractionation has an impact on primary and secondary porosity and permeability. Its effect on primary porosity is controlled by sorting and packing of grains, whereas secondary porosity is greatly controlled by the diagenetic alteration of feldspars to clay minerals. Off-axis and marginal facies have poor sorting and a higher proportion of K-feldspar, which indicates that these areas are likely to have lower porosity and permeability values compared to more axial locations.