Abstract

The reservoir rocks of the Tatagwa-Neprune-Colgate area are found in the Mulale "Vuggy" and "Marly". The "Vuggy" consists of repeated sequences of wackestone and pisolithic, oolitic and pe1letoidal grainstones and packsrones as well as centimetre laminated cruses. The cycles represent intermittently exposed shoals in the shelf crestal and outer shelf selling of the Mississippian depositional model for southeastern Saskatchewan. The "Marly", which is an earthy-appearing dolomicrite that shows signs of extensive bioturbation, was deposited in the inner shelf setting.

The reservoir quality of these rocks has been influenced by the original lithologies as well as by diagenetic processes that include: vadose alteration, burial cementation, dolominization and fracturing. The influence that these two factors have had on the geometry of the pore network can be demonstrated through empirically determined capillary pressure data. The two elements of capillary pressure analyses that are employed are R13 and pore throat sorting, both of which characterize the geometry f the flow paths in reservoir rocks. Graphs of these elements over the thickness of the core interval show that the dolomicrites of the "Marly" have a relatively uniform flow path geometry. By contrast, the "Vuggy" is more erratic with only thin intervals of uniformflow path geometry.

It is anticipated that understanding the vertical and lateral variations in [he pore networks of the reservoir in the study area, should assist in planning enhanced oil recovery schemes, particularly horizontal drilling programs.

Introduction

Seventy-five percent of the proven reserves in the Williston basin occur within the arcuate subcrop pattern of the Mississippian strata. The reservoirs in lhe Mississippian are controlled by the original depositional setting and the effects of the sub Mesozoic erosion surface (Kent (I)).

The Tatagwa-Neptune-Colgale oilfields are located in townships 5 and 6, ranges 15 and 16 wesl of the second meridian (Fig. I). They produce from the "Midale Trend" (Matiisen and Shehala(2)). A detailed core study of the field areas has established he relationship between reservoir rock and lithofacies and this has shown that the various lithofacies make up a framework of shelf margin and inner shelf deposits.

The lithologies and their depositional environments have an assortment of porosity types, permeabilities and areal extent and in an attempt to understand how these variations influence the reservoir quality of the field areas, we have employed empirical procedures understand the nature of the flow paths in the reservoir rocks. These empirical methods are developed from a large mercury injection capillary pressure (MlCP) database of Saskatchewan carbonate rocks and the principles behind the application are described by Kent et al. ((3)

REGIONAL STRATIGRAPHY

The Williston Basin paleobathymetry controlled facies distribution during the Mississippian time. The entire Mississippian succession can be thought of as a prograding, shallowing megacycle (Kent et al. (4)) interrupted by several smaller shallowing-upward cycles. In oldest Mississippian rocks in the central portion of the basin the succession includes deeper water carbonates that pass upward into shallow water deposits and eventually into continental rocks.

Post-Mississippian erosion has had a profound effect on hydrocarbon trapping potential of the presently southerly titled strata.

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