SEDIMENTOLOGICAL CHARACTERIZATION OF THE SEGO SANDSTONE (NW COLORADO, USA): A NEW SCHEME TO RECOGNIZE ANCIENT FLOOD-TIDAL-DELTA DEPOSITS AND IMPLICATIONS FOR RESERVOIR POTENTIAL

摘要

Marginal-marine deposits are typically excellent hydrocarbon reservoirs around the world. Certain depositional environments may provide natural, selective, physical processes that produce better reservoir rocks than others depending on associated energy, hydraulic regime, etc. With this paper we present new detailed sedimentological, petrographic, and grain-size data of ancient flood-tidal-delta deposits that provide means to characterize this environment in the ancient as well as to determine reservoir characteristics. In this study, we compare grain size, grain and cement composition, and the ratio of pore space to cement from thin sections between tidal, shoreface, and flood-tidal-delta facies of the Sego Sandstone in northwest Colorado. The intent of this comparison is to provide an evaluation of different physical processes and the degree of diagenesis related to different environments to better understand the relative controls on reservoir characteristics. We find that flood-tidal-deltas show a statistically larger average grain size than either shoreface or other tidal facies. Also, flood-tidal-deltas have a smaller ratio of pore-space to cement. Shoreface facies have a larger ratio of pore space to cement overall than that of flood-tidal-delta or other tidal facies. We propose that this may be due to chemistry and higher weathering rates in the backbay environment in contrast to the shoreface environment, where higher mechanical reworking may lead to selective removal of chemically unstable minerals. To test this hypothesis, we conducted statistical analyses of these petrophysical and grain size characteristics. Our results show that flood-tidal-deltas are characterized by clean upper-fine-grained cross-bedded sandstone that is well sorted and subrounded to rounded with a majority of landward paleocurrent directions and a presence of tidal bundles, mud drapes, and reactivation surfaces.