TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractThe 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.
The Minagish structure in southwest corner of Kuwait is a multi reservoir field. One of the potential reservoirs is the Mishrif formation. Developed as a limestone, sedimented in a mid-ramp environment, it generally consists of fine-grained packstones to wackstones that is highly bioturbated. The average thickness is about 300 ft with an average Net of 170ft in the upper layers. An average porosity value will be around 15% and permeability ranges between 0.001-17 mD. The oil in the Mishrif is highly viscous and production is normally enhanced by fractures in the upper Mishrif layers as they act as the main permeability conduit for the main storage below. The second Mishrif layer unit 2 (M9 & M8) is a fairly high porous peloidal packstone to grainstones sequence that is highly fractured at the upper 15 feet of the layer's "dual porosity system". The fracture corridors within the layer improve permeability, thereby making it a good potential for horizontal well placement. It was impossible to reach the observed production rates from matrix without one or two major fluid conductive fracture corridors. The main storage Mishrif layer unit 3 (M7 & M6) is within a 15% porosity layer that is 10–15 ft thick and contains a large volume of oil. Previous wireline image studies carried out identified these layers and their corresponding fractures, but because most of the studied wells were vertical, the fracture corridors could not be properly related to presence of faults in the field. This paper aims at showing how effective Logging While Drilling (LWD) resistivity images based on laterolog principle were useful in real time to identify the different layers within the Mishrif reservoir and establish the relationship between the fracture corridors and the intersected faults in a long horizontal well that posed a risk to data acquisition with standard wireline pipe conveyed logging. Introduction The Minagish (MN) field, in West Kuwait, is a North-South trending anticline (Fig.1) with hydrocarbons contained in six major reservoirs ranging in age from Early Jurassic to Late Cretaceous. The oil-bearing Mishrif/Rumaila limestone reservoir have been drilled and mostly appraised via wells dedicated to developing the Lower Cretaceous Minagish Oolite main reservoir which are estimated to account for up to 85% of the field's oil reserves. Mishrif is a tight layered, fractured reservoir of Cenomanian-Turonian (Upper Cretaceous) age developed over an asymmetrical anticline, dipping from east to west, the top of the formation was unconformable everywhere. Three main fault patterns were observed, defining three structural periods: EW and NS faults crossing the field in its central part during the Hith-Shuaiba period; mainly EW faults crossing the field in its central part during the Shuaiba-Ahmadi period; small NS faults in the north, EW trend en-echelon faults in the centre, NW-SE faults with a large extension in the south-west part of the field during the Ahmadi-Tayarat period. Most of the faults were generated during the Mutriba-Tayarat period (Turonian to Base Tertiary) but the trap formation of the Minagish field is post-Eocene. The recent Mishrif layering scheme divides the reservoir into 9 geological surfaces (time lines at field scale). There are two major sequence boundaries with exposure time, one at the top of the underlying Rumaila Formation and the other at the top of Mishrif (Fig.2). The lower part of the Mishrif consists of homogeneous mud dominated lithologies which is deposited in a slightly rimmed ramp environment. The upper part shows the occurrence of fine to very fine grained peloidal packstones to grainstones facies deposited in the more proximal shallower part of the ramp environment. The muddy sedimentation led to a very poor reservoir potential and the disappearance of porosity is due to heterogeneous cementation (nodules).
TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractThe Upper Cretaceous Mishrif reservoir in Minagish field is currently being developed by Kuwait Oil Company (KOC) using a horizontal drilling program. The Mishrif reservoir is approximately 300 ft thick across the field, with an average net pay of 170 ft in the upper layers. The reservoir porosity varies from 15 to 30%, and permeability ranges from 0.001 to 17mD. The first Mishrif horizontal well was drilled from west to east in the northern block of Minagish field. The well appears to have penetrated several generations of faults and associated fractures (early northwest to southeast, intermediate northeast to southwest, and possible late northwest to southeast. The production rate has been poor (approximately 500 bbl oil per day). An evaluation of the image logs indicated that only two of the reservoir layers appear to be fractured, whereas other layers are muddy and devoid of fractures, even near faults. To address the structural and stratigraphic uncertainties of the Mishrif reservoir, a high-resolution elemental chemostratigraphy study was performed on cored wells prior to additional drilling. The study produced a robust elemental zonation primarily based on variations in CaO/MgO, CaO/Sr, and MgO/Sr (carbonaterelated), SiO 2 /Al 2 O 3 and Zr/TiO 2 (detrital-related), and Br, S, Na 2 O, and Cl (diagenetic phases and/or formation waters). The study results were used to calibrate a portable laser-induced breakdown spectroscopy (LIBS) instrument, which was used for near real-time chemostratigraphy at the wellsite to assist in geosteering operations. The recent horizontal well penetrated a highly faulted section in Mishrif layer 2, with significant changes in dip related to faults. LIBS technology assisted in actively optimizing the well path in the porous limestone zone, only 5 to 10 ft thick, within this structurally complex regime. The distribution of possible fracture swarms and faults was reflected by abrupt changes in the geochemical profiles (MgO/CaO, S, [Ni+V+Fe 2 O 3 ]/Al 2 O 3 , Na 2 O, and Cl). This well achieved a new record for the longest horizontal drain hole in Kuwait.
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