The Evaluation of Reservoir Quality of Mishrif Formation in South and North Domes of Buzurgan Oil Field

Buzurgan field in the most cases regards important Iraqi oilfield, and Mishrif Formation is the main producing reservoir in this field, the necessary of so modern geophysical studies is necessity for description and interpret the petrophysical properties in this field. Formation evaluation has been carried out for Mishrif Formation of the Buzurgan oilfield depending on logs data. The available logs data were digitized by using Neuralog software. A computer processed interpretation (CPI) was done for each one of the studied wells from south and north domes using Techlog software V2015.3 in which the porosity, water saturation, and shale content were calculated. And they show that MB21 reservoir unit has the highest thickness, which ranges between (69) m in north dome to (83) m in south dome, and the highest porosity, between ( 0.06 0.16) in the north dome to ( 0.05 -0.21) in the south dome. The water saturation of this unit ranges between (25% -60%) in MB21 of north dome. It also appeared that the water saturation in the unit MB21 of south dome has the low value, which is between (16% 25%). From correlation, the thickness of reservoir unit MB21 increases towards the south dome, while the thickness of the uppermost barrier of Mishrif Formation increases towards the north dome. The reservoir unit MB21 was divided into 9 layers due to its large thickness and its important petrophysical characterization. The distribution of petro physical properties (porosity and water saturation) has shown that MB 21 has good reservoir properties. No.25(12) 2019 Journal of Petroleum Research & Studies (JPR&S) E90


Formation Evaluation and Log Analysis:
Formation evaluation can be generally defined as the practice of determining both the physical and chemical properties of rocks and the fluids they contain. The objective of formation evaluation is to locate, define, and produce from a given reservoir by drilling as few wells as possible. To this end, oil companies utilize a variety of formation evaluation methods [2].
Wireline logs are one of the many different sources of data used in formation evaluation.
However, due to accurate depth determination and near proximity of receiver to formation, wireline logs occupy an important position in formation evaluation. Logging is a very small, but very important, piece of the larger puzzle. The decision to plug or complete a well is often based upon the logs response and hence a proper and accurate acquisition and analysis of these data is a must [2].

Area of study:
Buzurgan oil field is located in South -Eastern part of Iraq close to Iran boundary, 40 Km North East from Amara. The Mishrif Formation represents an important reservoir in southern Iraq.

Petrophysical Properties:
Petrophysics is the study of the chemical and physical characteristics that define the habit and presence of rocks and liquids. Logging helps to describe physical characteristics of rocks for example, porosity, lithology, permeability and pore geometry. Logging data are utilized to detect pay zone to fix thickness and depth of intervals, to distinct amongst gas, oil or water in reservoir and to estimate oil reserve [3].
Petrophysical properties that are discussed in this text include:

Water saturation
• Permeability

Clay Volume:
Because clay is typically further radioactive than carbonate, GR tool will be suitable candidate to calculate amount of clay in permeable reservoir. The shale volume is expressed as a decimal fraction or percentage is named Vshale. The measurement of the Gr index is the principal stage required for define the shale volume by GR log [3]. Where: : is the gamma ray index, : is the gamma ray log reading in zone of interest, API units, : is the minimum gamma ray reading in clean zone, API units, and : is the maximum gamma ray reading in shale zone, API units.

Porosity:
Porosity is the ratio of spaces to the total volume of rock. Porosity is signified as a percentage by the Greek letter phi, Ø [3]. Porosity logs Neutron, Density, and Sonic are E93 mainly related to porosity, and they also influenced by lithology, formation matrix,kind of porosity and degree of shaliness and kind of liquid existing in the pores [4].
The neutron -density logs provide the best combination to identify gas zones and determining their porosity. The combination also provides porosity in complex lithology and volume of shale for shaly formation evaluation.
Porosity logs include sonic logs, density logs, and neutron logs. The sonic log records matrix porosity, whereas the nuclear logs (density or neutron) determine the total porosity [5].
For reservoir characterization it is important to distinguish between: 1. Total porosity (the fraction of bulk volume occupied by the total pore space or the space not occupied by solid components).
2. Effective porosity (the fraction of bulk volume occupied by interconnected pore space allowing fluid flow).
Porosity should be calculated from the density log using the equation [6]: Where, : is the density-derived porosity, fraction.
: is the matrix density, [whose value is 2.71 gm/cc for limestone and 2.87 gm/cc for dolomite].
: is the formation bulk density, gm/cc.
: is the fluid density [For fresh water mud = 1 gm/cc, for salt water Mud =1.1gm/cc].

Total porosity:
Total porosity is defined as the ratio of the volume of all the pores to the bulk volume of a material, regardless of whether or not all of the pores are interconnected [7]: Effective porosity: Effective porosity is the ratio of the volume of interconnected pore to the total volume of reservoir rock [7] .

Secondary porosity:
Secondary porosity is porosity formed within a reservoir after deposition. In Mishrif reservoir, secondary porosity is too little or negligible, vuggy or fracture secondary porosity can be calculated by secondary porosity index (SPI) [7].
The secondary porosity is the result of geological processes (diagenesis) after the deposition of sediments [8]. It includes vugular spaces in carbonate rocks created by the chemical process of leaching, or fracture spaces formed in fractured reservoirs [9].
The intervals of higher secondary porosity mean existence the effect of digenesis processes on the porosity of Mishrif formation such as dolomatization and dissolution.
On other hand, has been distinguished an irregular alternation of two type of vuggy porosity by borehole images, it is the separate and touching vuggy porosity. This porosity is either opened or filled partially or completely by calcite cement. The interconnected only through the Inter particle pore network refer to the Separate vugs, though touching vuggy porosity shape an interconnected pore framework [10].

Fluids in the pore space: Saturation and bulk volume fluid:
Porosity represents the ratio between pore volumes related to the bulk rock volume.
Saturation represents the ratio between volume of pores which occupied by a fluid related Saturation is given as a dimensionless fraction or as a percentage. Saturation theoretically has the lower bound at zero (or 0%) and the upper bound at one (or 100%). A reservoir hosting the fluids water, oil, and gas is characterized by three saturation terms; their sum must be 1[11]: Fluid saturation can be determined: a. From cores, plugs, or samples (direct determination by fluid extraction, or capillary pressure measurements).
In addition to the parameter "saturation", the parameter "bulk volume of the fluid" is used.
Bulk volume of a fluid relates the volume of that fluid to the rock bulk volume. The bulk volume of, for example, water is therefore given by: [11]
In a (water wet) porous rock, the water, depending on its interaction with minerals and bonding type, is present as [11].
1. Free movable water in the pore space (bulk volume movable VM).

Capillary bound water, connected with the grain surface (bulk volume immovable BVI).
3. Clay-bound water (CBW) with its strong clay-water effects.

Water Saturation:
Measuring pore volume in a rock that is engaged by formation water, it is signified as decimal portion or as percentage and has the symbol (Sw). Water saturation of a reservoirs uninvaided interval is calculated through the Archie's equation [3].
is the true formation resistivity.
F is the formation resistivity factor.
n. is the saturation exponent, m: is the cementation factor, a: is a constant.

Hydrocarbon Saturation:
The hydrocarbon saturation is the quantity of pore volume in a stone which engaged by oil, typically detected by the difference amongst unity and water saturation, though the residual hydrocarbon saturation is the difference amongst unity and water saturation in flushed interval [14]: