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Source-Rock Plays and Sequence Stratigraphy: What Makes the Best Part of the Best Plays?

Hart, Bruce
ConocoPhillips, Houston, TX.

Stratigraphic analyses of a variety of mudstone-dominated units demonstrate that the best production from source-rock plays (i.e., the reservoir is also the source rock) is usually from thermally mature, pelagic-rich strata that can be assigned to the transgressive systems tract and condensed section (TST/CS). They are often referred to as "black shales", although clay minerals can form 20% or less of the rock. A high TOC content (typically Type I/II) made these strata potential source rocks, and biogenic silica and/or carbonate produced "brittle" rock that is suitable for hydraulic fracture stimulations. Where deposition from suspension dominated, relatively thin (< 10 m thick) stratigraphic units can be correlated over several to many 10s of km and generally show draping to divergent/convergent geometries. Rock properties are therefore unlikely to change significantly over the length of a typical horizontal completion (~ 1 km) in these suspension deposits, unless other complications (e.g., fractures) are present. However, changes in thickness, mineralogy, thermal maturity or other stratigraphic/geologic parameters over distances of several km to 10s of km will affect hydrocarbon generation, storage and production.

The lithologic and stratigraphic characteristics of the TST/CS strata differ considerably from progradational mudstones of the highstand systems tract (HST) and the lowstand systems tract (LST). The latter are characterized by higher clay contents and more detrital silicate/carbonate as silt or coarser grain sizes. The clay content generally makes these shales less brittle than the TST/CS units. These HST/LST mudstones have lower TOC contents than the TST/CS mudstones, and the organic matter is relatively enriched in terrestrial (Type III) constituents. These strata tend to be more heterolithic than the TST/CS mudstones, and may be arranged into submarine failure complexes, prodelta lobes or other stratigraphic features that can show rapid lateral variations in lithology and thickness. These characteristics generally make these HST strata poorer source rocks and more complex to development using horizontal wells than TST/CS mudstones.

Although the descriptions and interpretations presented above have general applicability (i.e. they are the "simple case"), these concepts commonly need to be modified to account for the wide variability in depositional setting, depositional history, and geologic age, represented by gas-productive mudstones.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012