Geosteering in a complex lithology environment of Wara sand using the Multi-Function Logging While Drilling Tool, Minagish Field of Kuwait
Authors: El-Gezeery Taher M.N. , Al-Saqran Fawaz , Archibong Ekpo Ita , Oyeyemi Oluwafemi , Chimirala Vivekanand 
Published: December 2009

The Wara Sandstone reservoir in the Minagish Field of Kuwait Oil Company is a complex deposition of a typical pro-deltaic environment. The sedimentation starts from tidal marine deposition containing beach sands and lagoonal facies that are subsequently overlain by fluvial channel sands. The bases of the channel sandstones have in many cases an erosional contact that cuts into the underlying sequence. The marine sandstones are of much finer grains and poorer quality than the overlying channel sands that are generally coarser in grain size, thereby having better porosity and permeability. On top of the channel sequence, a marsh overbank or lagoonal environment overlay and are characterized by silts and coaly layer sequence. They are also finalized by a subsequent deposition of progradational coastal marine silts and fine sands. All these deposits find their high stand in the overlying Ahmadi formation.

The sediment sequence is repeated up to four times and zero to four channels may develop and intercepted by drilled wells depending on the well location within the field.

Within this channel sand bodies are different lobes that contain varying degree of minerals from glauconite to anatase, pyrite and hematite with other cement materials like calcite. The matrix materials in the more shaly intervals are predominantly illite and glauconite. Glauconite occurrence was either deposited via transportation (detrital) or generated during early diagenesis to act as cement materials. Glauconite cement has a strong impact on reservoir producibility and since it is present throughout the entire sequence of depositional events, it plays a great role in the petrophysical evaluation of the reservoir. Being able to geosteer within the sand bodies will require proper understanding of the depositional environment and thus requires discriminating intervals with relative abundance in glauconitic grains as they impact reservoir quality.

It has been proven from previous core studies that the more the detrital glauconite occurrence, the greater is the relative abundance of glauconite cement as well. In this paper we propose the use of increase in the presence of iron (Fe) dry weights with its associated Titanium from the multi-function logging while drilling (LWD) tool to discriminate the relative abundance of glauconite in the reservoir sand bodies, thereby characterizing the lobes. We will also highlight the benefits derived from using the formation sigma in real time for petrophysical interpretation and its usefulness in deciding intervals to perforate in horizontal well drilling.

Introduction

The Minagish field is located in the West Kuwait (Figure 1) is a north-south trending anticline with hydrocarbon contained in six major reservoirs ranging in age from early Jurassic to late Cretaceous. The field was discovered in 1959 and went on production in 1961. Its primary reservoir is the early cretaceous (Neocomian) Minagish Oolite (MO) formation that is contributing a major part of the total field production.