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Abstract: Abstract: Diagenetic Control of Reservoir Quality in Arc-Derived Sandstones--Implications for Petroleum Exploration

William E. Galloway

The potential of a sandstone to serve as a reservoir for producible hydrocarbons is closely related to its diagenetic history, which, in turn, is dependent on the composition of the basin sedimentary fill. Depositional basins formed marginal to contemporaneous or precursor andesitic volcanic arcs at convergent plate margins receive large volumes of detritus reworked from the arc or its tectonically uplifted plutonic roots. Arc-derived sandstones of the Bristol, Gulf of Alaska, Queen Charlotte, and Grays Harbor-Chehalis basins (northeast Pacific) form compositionally similar suites dominated by plagioclase feldspar and volcanic-rock fragments. Diagenesis of these mineralogically immature sandstones at shallow to intermediate depths of burial (< 15,000 ft or 5,000 m) pro uces a characteristic sequence of authigenic cements, including (1) local early calcite pore fill, (2) clay coats, (3) laumontite or phyllosilicate pore fill, and (4) late stage calcite pore fill or replacement and siliceous overgrowths. Each diagenetic phase reduces reservoir porosity and permeability, and correspondingly increases bulk density and interval velocity of sandstone sections. Well-developed clay coats in fine-grained sandstones reduce maximum permeabilities to a few tens of millidarcys; samples containing stage 3 laumontite or phyllosilicate typically have less than 10 md permeability.

Onset of diagenetic stages 2 and 3 is in part temperature controlled. Implications of this relation are considerable because generation of oil is also temperature dependent. Generation of oil in the optimum thermal window lies dominantly within rocks characterized by stage 3 to 4 diagenesis and consequently with poor reservoir quality. Prediction of diagenetically controlled economic basement is necessary for accurate assessment and systematic exploration of convergent arc-related plate-margin basins.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado