Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Deciphering Depositional Controls on Shale-Gas Reservoir Distribution and Resource Density Using Detailed Sedimentology and Sequence Stratigraphy
(1) ExxonMobil Exploration Company, Houston, TX.
(2) ExxonMobil Upstream Research Co, Houston, TX.
Detailed examination of a wide range of physical, biogenic, and chemical attributes indicates that shale reservoirs such as the Barnett, Marcellus, Haynesville, Eagle Ford, Bakken, etc., accumulated discontinuously in intra-shelf basins in various depositional environments with significant changes in primary biogenic production, bottom-energy and oxygen levels, and sediment accumulation modes and rates. Rock properties that influence reservoir behavior vary in systematic ways that can be deciphered using an integrated sequence-stratigraphic approach.</p>Scour surfaces, wave and current ripples, graded beds, fossil and pyrite lags, and wave-enhanced sediment gravity flow bedsets record benthic energy levels episodically capable of transporting sand-sized grains. Some intervals show laterally extensive erosion with relief up to 10 m over 5 km spans. In-situ body fossils in some intervals indicate significant and recurring periods of benthic oxygenation— benthic foraminifera as well as articulated siliceous sponges and epi-benthic and shallow-burrowing bivalves. The epi- and endo-bionts are relatively low in diversity and of a type adapted for life on dysoxic, soupy substrates. They co-occur with ichnofossils and sedimentary structures that indicate increased benthic oxygen and energy levels.</p>Recognizing flooding surfaces, sequence boundaries, and systems tracts is key to identifying stratigraphic patterns of reservoir potential. Content of such properties as biogenic silica, TOC/HI, clay, and carbonate that control reservoir character varies systematically. For example, Barnett Shale reservoir quality and resource density can be tied directly to systems tracts. Strata interpreted as lowstand system tracts also have significant development of porosity due to enrichment, preservation, and maturation of organic material along with increased brittleness from enhanced sponge and radiolarian production and accumulation. Reservoir potential is greatly diminished within transgressive and highstand systems tracts due to increased clay-mineral content. These system tracts are, however, important elements of the play as they provide seals and frac barriers for well completions. The development and distribution of depositional facies and system tracts are controlled by the bathymetry and lateral configuration of the depositional basin. In this example, positive topography as well as detrital sediment trapping benefits reservoir potential in lowstand systems tracts.