Transgressive Reworking and Its Impact on Sandstone Porosity Improvement: The Vivian Formation, Marañón Basin, Northern Perú

Abstract

The craton-derived, quartz-rich sandstones of the Upper Cretaceous Vivian Formation are the most important reservoirs in the prolific Marañón foreland basin of northern Perú. Porosity versus depth analysis for the dominantly fluvial Vivian sandstones shows a simple trend of decreasing porosity with depth, which suggests that overburden stress is the dominant factor that determines porosity reduction in the basin. However, anomalously high porosities at significant depths showing a substantial diversion from the depth versus porosity regional trend were identified in few wells. Gamma ray (GR) - based log signatures indicate that those intervals with anomalously high porosities are identified in thin, fining up (dirtying up) successions resting on slightly coarsening up (cleaning up), blocky or funnel-shaped GR electrofacies, dominant on the fluvial Vivian sandstones. The limiting surface that marks the change from fluvial/shallowing-upward strata below to deepening-upward, transgressive strata above is considered a maximum regressive surface (MRS). The interval bound between the MRS (coplanar with the initial flooding or transgressive surface, ITS) and the maximum flooding surface (MFS) defines the transgressive systems tract (TST).

TST thickness variation along a depositional dip direction can be interpreted as an estimate of transgressive reworking. This process is effective when rates of net deposition and relative sea level change are in equilibrium, allowing extensive reworking of underlying sands. SE-NW- oriented depositional dip cross sections for the Vivian Formation illustrate the transition from a thin proximal sand-rich TST to thicker, more distal sand-poorer TST. Depending on the location along the depositional dip, the MFS may be located very close to or almost coincident with the ITS (=MRS) in proximal parts of the basin and get decoupled significantly basinward leading to a thicker TST. Porosity values in the early TST sandstones may be significantly higher (up to 8-10%) than the fluvial blocky sands, suggesting transgressive reworking of the underlying sand-rich deposits. This porosity increase detected in the thin, transgressive sandstones of the Vivian Formation indicates that the decoupling of the ITS and MFS can be used as an estimate of wave reworking during transgressions.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018