Monday, September 30, 2013

Highlights—Unconventional Unconventionals?

“Unconventional” petroleum reservoirs represent important, but still enigmatic, depositional systems.  In this paper, Egenhoff and Fishman describe and interpret organic-rich mudstone of the upper member of the Upper Devonian-Lower Mississippian Bakken Formation in the intracratonic Williston basin of North Dakota. Detailed sedimentological characterization including ichnology and analysis of TOC reveals several facies associations, and facilitate a conceptual model for processes governing the deposition of these strata. The results show spatial and temporal changes in facies associations dominated by bed-load transport, and ichnofossil characteristics that demonstrate a distinct gradient from higher to lower diversities towards the basin center. These data are interpreted reflect deposition of upper Bakken shales in a dysoxic environment, on a highly differentiated deep shelf that was still influenced by storm-induced currents. This interpretation that the basin was dysoxic (not anoxic) challenges the conventional model of basins in which many unconventional reservoirs occur, and may be applicable to other black shale units that form world-class source rocks worldwide.

Friday, September 27, 2013

A Look Back…50 Years: Partisan Battles in Mollusks

In politics, “right versus left” is a common theme. Although it is less frequently that this motif is carried to sedimentologic matters, it was 50 years ago that Kornicker et al. explored the question: Why do right or left valves occur in excess abundance in recent and ancient sediment accumulations? Their study explored the distribution of clam valves on the recent beach of Mustang Island, Texas. The results indicated that a differentiation is statistically significant only in valves larger than 69.5 mm, interpreted to represent the influence of “high energy surf” in separating valves.  Shell attributes such as weight, ornamentation, and projections did not impact the distribution of valves. And, like politics, neither right nor left own a monopoly on boring, and it is not a control on differentiation.  

Factors Affecting the Distribution of Opposing Mollusk Values by Louis S. Kornicker, Charles D. Wise, Juanita M. Wise, Journal of Sedimentary Petrology, v. 33, p. 703-712.

Monday, September 23, 2013

A Look Back…20 Years: Microbes and Carbonates

Geomicrobiology is a rapidly growing field in sedimentary geology.  Yet, 20 years ago this month, Folk presented numerous SEM images that illustrated micro-organisms associated with carbonate sediment and cement from Holocene travertines, hardgrounds, and ooids, and from rocks as well.  The data presented intriguing evidence for the role of micro-organisms in precipitation of carbonate minerals, leading him to conclude “the minute interface between bacteria and carbonate petrology may be lilliputian in scale but are conceivably gargantuan in importance….”

SEM Imaging of Bacteria andNannobacteria in Carbonate Sediments and Rocks by Robert L. Folk, Journal of Sedimentary Petrology, v. 63, p. 990-999.

Wednesday, September 18, 2013

A Look Back…75 Years

Rivers obey the laws of physics and flow downhill, commonly eroding elevated regions and flowing towards the ocean.  Seventy-five years ago, Edelman described the mineralogical character of sediment in rivers in the Netherlands (grain size characterization was described as “unjust”).  The data reveal that the mineralogy (and grain size) changes markedly downstream; changes that he interpreted to reflect a North Sea source for downstream segments.  He pointed out that rivers in the Netherlands “do not succeed in transporting even one single sand grain to the neighboring North Sea… exactly the opposite… of what inference teaches.”  [Although he did not explicitly state it, this dynamic is due to tides in the estuaries.  See HERE and HERE for more information.]  Edelman suggests that these data “warn us not to apply the pattern-like conceptions of rivers and their sediments…without a critical examination.”

Petrology of Recent Sands of the Rhine and the Meuse in the Netherlands, by C. H. Edelman, Journal of Sedimentary Petrology, August 1938, v. 8, p. 59-66.

Tuesday, September 3, 2013

Highlights—Graphic Detail of Graphic Logs

Geophysical logs are a primary means for interpretation of sedimentology and stratigraphy of subsurface aquifers and reservoirs. This contribution by Farrell et al.  illustrates the techniques and application of a process-based method of graphic logging and stratigraphic interpretation, independent from composition, cementation, and geologic environment. The utility of this texturally-based classification of clastic sediment is evidenced by four examples of shelf to shoreface successions, including steps of identifying rock units (facies) independent of composition, and interpreting environments, facies associations, boundaries and systems tracts. In these successions, gamma logs indicate grain size and spikes in radiation are associated with coarse lags at sequence boundaries, not high mud content. This process-based technique, interpreted to integrate facies analysis and sequence stratigraphy, should be broadly applicable to capturing and interpreting heterogeneity in reservoir or aquifer quality in subsurface systems.

Graphic logging for interpreting process-generated stratigraphic sequences and aquifer/reservoir potential: with analog shelf to shoreface examples from the Atlantic Coastal Plain Province, U.S.A. by Kathleen M. Farrell, W. Burleigh Harris, David J. Mallinson, Stephen J. Culver, Stanley R. Riggs, John F. Wehmiller, Jessica Pierson Moore, Jean M. Self-Trail, and Jeff C. Lautier