ABSTRACT: Ancient and Modern Deltas: Some Perspectives and Stratigraphic Implications
CHAPLIN, JAMES R.
Fluvial-deltaic
deposits
are the Mid-Continent's most important hydrocarbon reservoirs.
Deltaic
facies show enormous lithic variability with multiple opportunities for
facies changes in short time and space intervals.
Some important observations
to note in the recognition and interpretation of deltaic
facies include
the following: 1) There is a lack of type well logs tied into subsurface
reference sections and into updip reference surface sections. 2) Earlier
workers were less successful in describing
deltaic
facies, because they
used a low-density data base of primarily electric log not core descriptions
or outcrop information. 3) Log patterns are representative of major
deltaic
depositional facies, but in no way do they indicate all of the possible
facies variations. 4) It is significant to understand not only the physical
processes of delta formation, but also the gross sedimentology of the sand
bodies. 5) There is a lack of regional chronostratigraphic correlations
of
deltaic
sandstones to convincingly demonstrate if the
deltaic
facies
are temporally equivalent to the reservoir sandstones. 6) Existing modern
deltas do not convincingly describe the
environments
of deposition of ancient
counterparts. 7) Classifications of delta systems by earlier workers may
not always be applicable because the study area is too small to allow for
the determination of such criteria as the overall shape of the inferred
delta. 8) Analysis of most major
deltaic
hydrocarbonproducing sandstone
reservoirs in the Mid-Continent has not yet been conducted within a rigorous
sequence stratigraphic framework. 9) Some significant channelized bodies
are marine bodies resulting from the extension of delta systems into offshore
areas during sea-level lowstands. (10) Different stratigraphic correlations
yield different interpretations; correlations based on lithologies do not
give the same sandstone-body geometries or interpretations as those based
on boundary discontinuities.
An understanding of all of
these complex stratigraphic relationships is absolutely necessary in predicting
reservoir occurrence and quality in deltaic
facies, particularly in less
explored parts of foreland basins.
AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas