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Abstract: Five Things Geophysicists Should Know About Shale Plays

Five Things Geophysicists Should Know About Shale Plays

The Shale Revolution caught geophysicists off guard. Shales had been studied for a variety of reasons (e.g., relationships between velocity, compaction and pore pressure) but not as low-porosity reservoirs that show vertical heterogeneity at all possible scales. Consequently, many geophysicists have framed shale-play imaging problems using inappropriate tools and paradigms. In this presentation, I present five characteristics of shale plays that should enable improved geophysical analyses.

  1. The term “shale play” has become meaningless. Originally intended to describe gas production from fine-grained source rocks (“source-rock reservoirs”), the term is now applied almost indiscriminately to production from many types of low-permeability rock (e.g., shaly sandstones, carbonates).
  2. Source-rock reservoirs aren’t clay dominated. Hydraulic fracturing is needed to establish commercial production from these rocks. Clays make the rocks ductile and harder to fracture. As such, the clay content of shale plays is generally less than 50%. The remainder of the rock is usually composed of fine-grained calcite and/or quartz, organic matter and other minerals.
  3. Links between VTI anisotropy and clay or organic content are not straightforward in source-rock reservoirs. Scanning electron microscopy often reveals textures that are incompatible with the conceptual models used to develop mathematical models of shales.
  4. HTI anisotropy is complicated by natural fracture geometries. Aligned natural fractures generally combine with bedding to produce systems that are best described as orthorhombic. In some cases, multiple fracture orientations produce systems that are effectively isotropic.
  5. Integration of geophysical and geological data and concepts is needed to significantly advance geophysical research on shale reservoirs. This effort will allow geophysicists to define, for a specific shale, which assumptions are reasonable, which analogs are appropriate, what appropriate ranges of properties are, etc.

Distinguished Lecturer

Bruce

Bruce Hart

AAPG-SEG Distinguished Lecturer

Research Scientist

Equinor, Austin

Video Presentation

Abstracts

  • 41229 The Ice Age and the Giant Bakken Oil Accumulation The USGS estimated (2013) that the Late Devonian to Early Mississippian Bakken Formation holds in excess of 7 billion barrels (~1.1 billion m3) of recoverable oil, making it one of the top 50 largest oilfields in the world. Most of the production comes from shallow-marine sandstones of the Middle Bakken Member that are directly over- and underlain by extremely organic-rich shale source rocks (Upper and Lower Bakken Shale members respectively). Although not oil-productive everywhere, the Middle Bakken forms a relatively sheet-like unit that covers an area of over 200,000 square miles (~520,000 km2) of the intracratonic Williston Basin. The vertical juxtaposition of shallow-marine reservoir and more distal source rocks over such a large area, without intervening transitional facies, is unusual from a stratigraphic perspective. One possible explanation would require global fluctuations of sea level to drive geologically rapid and extensive shoreline movements in this relatively stable basin. Forced regression associated with falling sea level could explain the lack of transitional facies (e.g., inner shelf) between the distal Lower Bakken Shale and the overlying Middle Bakken (a sharp-based shoreface). Subsequent sea-level rise would have caused rapid and extensive transgression, leading to the observed stratigraphic relationships between the Middle and Upper Bakken members. But what could have caused the changes in sea level? A considerable body of evidence points to a Late Devonian-Early Mississippian ice age that covered portions of Gondwana (e.g., parts of present-day Brazil) that were situated close to the paleo South Pole. This ice age consisted of more than one glacial/interglacial cycle and was probably triggered by massive removal of CO2 from the atmosphere by land plants and organic-rich shales. Some evidence indicates that at least 100 m of sea-level drop took place during one of the Famennian glaciations, which would have effectively drained the Williston Basin and induced major shoreline progradation. Melting of the ice sheets would have caused transgression and reflooding of the basin and deposition of the Upper Bakken Shale. Other basins around the world record similar evidence for glacioeustacy near the Devonian-Mississippian transition. The glacial/interglacial cycles are expressed differently from basin to basin, reflecting the interplay between fluctuations of global sea level and each basin’s history of subsidence and sediment supply. Ice Age and the Giant Bakken Oil Accumulation http://www.aapg.org/career/training/in-person/distinguished-lecturer/abstract/Articleid/41229/ice-age-and-the-giant-bakken-oil-accumulation
    Ice Age and the Giant Bakken Oil Accumulation

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