--> Abstract: The Role of Sequence Stratigraphy in Unraveling and Applying the Complex Controls from Mudstone Reservoir Properties, by Kevin M. Bohacs and Ovidiu R. Lazar; #90078 (2008)

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The Role of Sequence Stratigraphy in Unraveling and Applying the Complex Controls from Mudstone Reservoir Properties

Kevin M. Bohacs and Ovidiu R. Lazar
ExxonMobil, Houston, TX

Productive mudstone reservoirs commonly have significant biogenic content of both source-prone organic matter and brittle lithofacies, as well as large clay content, parallel-laminated fabrics, and early diagenetic cements. These attributes vary at the cm-scale vertically and at the km-scale laterally in systematic ways that can be deciphered using process-based models within a sequence-stratigraphic framework.

Paleozoic, Mesozoic, and Cenozoic examples (including New Albany, Barnett, and Mowry Shales, Monterey and Sisquoc Fms) exhibit major shifts in reservoir properties at sequence boundaries, flooding, and downlap surfaces that can be recognized independently on seismic, log, core, and thin-section data. They also show systematic vertical and lateral variations in those properties at the parasequence- and parasequence-set scale. Reservoir facies tend to occur in discrete packages that are diachronous across a basin, making it essential to employ the various physical sequence-stratigraphic surfaces to correlate appropriately and to decipher the distribution of reservoir potential.

Biogenic content is controlled by complex, nonlinear interactions among rates of production, destruction, and dilution. Early diagenesis is similarly influenced by rates of biogenic-matter input and burial, and by sea-floor and upper-sediment-column conditions, along with rates and character of terrigenous inputs. Few mudstone reservoirs are dominated by only one of these factors-- most record a variety of interactions of all variables that change vertically and laterally. In our examples, we interpret that major shifts in reservoir properties at the sequence scale record factor changes of 2 to >5 in primary biogenic production rates, 1.5 to >11 in destruction rates, and 10 to >50 in dilution rates. At the parasequence scale, factor changes of 2 to >30 occur laterally over tens of kms.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas