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