“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.
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.
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….”
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.”
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.
