Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sequence-Stratigraphic Controls on Sandstone Diagenesis – An Example from the Williams Fork Formation, Piceance Basin, Colorado
(1) Geological Sciences, University of Colorado at Boulder, Boulder, CO.
Whether the spatial and temporal distribution of diagenetic alterations in fluvial sandstones are predictable within a sequence stratigraphic framework remains an important research question. This study investigates that question using the Cretaceous Williams Fork Formation. The Williams Fork Formation has been interpreted to contain 5 composite sequences, each composed of fluvial and floodplain deposits, with shoreline and coastal material to the east.
Facies and sequence stratigraphic framework controlled the spatial and temporal distribution of most diagenetic alterations in the Williams Fork. Eogenetic alterations are particularly facies and systems tract specific and include: (1) mechanically infiltrated clays, particularly in channel and crevasse-splay sandstones towards the top of the LST at the base of the upper Williams Fork, and at the base of all HSTs; (2) pseudomatrix, developed by mechanical compaction of mudstone intraclasts, occurred mainly in channel sandstones at the top of HSTs and led to porosity and permeability deterioration; (3) Authigenic chlorite, formed in TST and HST of the upper Williams Fork in association with mafic volcanic clasts, detrital Fe-Mg silicates, and dissolution of syngenetic berthierine. This grain-coating chlorite is associated with near-shore shallow-marine deposits, tended to inhibit the development of quartz overgrowths, and is a factor contributing to the preservation of primary porosity; (4) calcite and dolomite are associated with floodplain facies that developed on top of crevasse-splay and channel sandstones of TSTs in the lower and upper Williams Fork.
Porosity and permeability were extensively degraded during mesodiagenesis. With the exception of quartz overgrowths, mesogenetic alterations tend to be independent of facies and system-tract position. These include: (1) late carbonate (calcite and much less dolomite) replaces framework grains and forms cement, and occurs in all depositional facies and systems tract sandstones within the middle and upper Williams Fork sandstone; (2) illite and illite/smictite, which are abundant in secondary pores in all systems tracts, formed with albitization of detrital Kfeldspar; and (3) quartz overgrowths, which are most abundant in TST sandstones due to the general absence of infiltrated, grain coating, and pseudomatrix clays. The abundance of infiltrated clays and pseudomatrix in LST and HST sandstones inhibited quartz overgrowths.