Geology Paper on Nondeltaic, Shallow Marine Deposits and Reservoir

The continental shelf refers to the region between the continental slope and the shoreline, on the continental margin. While the continental shelf width and slope vary across geographic areas, the average width of the continental shelf is 75km and the gradient is 1.7 m/km. Since the depositional processes occur differently, the deposits vary significantly. For instance, deposition at the mouth of the river varies from the deposition where major rivers debouch into the ocean. The above depositional processes result in a deltaic shoreline and “interdeltaic” (nondeltaic) shoreline and shelf. This chapter explores nondeltaic shoreline to shelf-edge deposits and reservoirs.

Shallow Marine Processes and Environments

Nondeltaic shoreline environments receive sediments from various sources. Sediment can be deposited at the mouth of a river then carried alongshore by waves and currents or sediment may be transported from offshore seafloor to the land to form beaches. Water particles move in a circular motion but friction between the seafloor alters the movement to an elliptical pattern. These water particles movement transport and deposit sediment with coarse grains towards the land, resulting in different sedimentary structures. Tides also greatly influence sedimentation patterns nearshore. In events of stronger flood tides of open tidal channels, small flood-tidal may occur in the landward direction while strong ebb tide may result in ebb-tidal deltas in the seaward direction of tidal channels.

Nondeltaic Shallow Marine Deposits Within A Sequence Stratigraphic Framework

Rapid transgressions of the shoreline normally attribute to nondeltaic shoreline and shallow marine deposits in a sequence stratigraphic framework, which is part of the transgressive systems tract (TST). TST involves landward migration of the shoreline and the upward movement of shelf waters, resulting in shoreline reservoir sands. Shoreface environments are also associated with the offshore or open-shelf (seaward) environments as well as swash, beach, and dune environments. As the water depth increases offshore, sediment grains also become finer.

Shoreline and Shallow Marine Deposits

  1. Shelf, shoreface, and, or valley fill sandstones

The “shoreface” and “shelf” concepts often occur at the center of the debate of the origin of long, linear sand particles that mostly occur beneath marine shale therefore forming excellent stratigraphic trap potential. Snedden and Bergman also suggested “incised valley fill’ as the origin of some of the sand bodies. Cretaceous Shannon, popular sandstone, is believed to have been shell ridges that were deposited far away on the continental shelf and were therefore disconnected from the paleoshoreline. This interpretation, as opposed to the static sea level during deposition, was widely accepted due to the establishment of sequence stratigraphy concepts in relation to sea-level fluctuations.

  1. Significance of the Origin of Deposits

Nondeltaic sandstone sequences can extend on a wide area laterally, which makes them good reservoirs on the subsurface. Since the internal connectivity of reservoir sandstones differ significantly among shoreface, shelf, and incised valley, it is important to study the reservoir characterization and performance.

Shoreface/Shallow Marine Reservoirs

  1. Hartzog draw field

The Hartzog draw field, which is located in the Powder River Basin, is the biggest Shannon Sandstone field. It is averagely 35 km long and 3 km wide. It was discovered in 1975, leading to increased primary oil production in 1977.

  1. Terry Sandstone, Denver Basin, Colorado

This sandstone, located in Hambert-Aristocrat field, is a series of stacked shoreface sandstones which are vertically separated by shales. The Terry Sandstone is located in the Denver Basin, Colorado and it produces both oil and gas.

  • Some Midcontinent Pennsylvanian Sandstones, Oklahoma, United States

Pennsylvanian sandstones in Oklahoma, which are hydrocarbons, have both vertical stratigraphy and a lateral distribution. The vertical stratigraphy is chracterised by a coarsening-upward log pattern overlain by a fining-upward pattern.

  1. Early Tertiary Shallow Marine Sandstones, Mexico

After gas was discovered in Pliocene sandstones in Cantrell, Mexico, researchers concentrated on analyzing these same aged rocks in other geographic regions basing on core, well log analysis and biostratigraphy. They found out that all these sandstones have a blocky log character and also contain marine fossils.

Barrier-Island Deposits and Reservoirs

  1. Complex processes and deposits

The variety of processes involved in the formation of barrier-island deposits make them complex. They are influenced by ocean currents and shallow water waves which pass alongshore and deposit sand on the beachfront. On a longer time frame, complex barrier-island deposits are impacted by fluctuations in the sea level.

  1. Bell creek and recluse fields, Montana and Wyoming, United States

The above two are mature and well-studied fields which occur in a series of Cretaceous Muddy Sandstones reservoirs on the joint Montana State border and Wyoming. Since these fields have been grilled extensively, they are mostly used to study the complexities of production and geology in barrier-island reservoirs. While both Recluse and Bell Creek fields consist alongate sandstone, sand bodies tend to be less continuous in Recluse fields, hence the compartmentalization. Compartmentalization mostly occurs in highly structured wells with water compared to oily wells. Recluse field sandstones texture varies from fine to medium-grained. Fine grains indicate low porosity and permeability.