Integrating Well Logs and Core Data for Better Reservoir Characterization of Mamuniyat Formation, Murzuq Basin, Libya

Abstract


Introduction
The Murzuq Basin is one of the numerous intra-cratonic basins on the North African Platform, covering an area of around 350 000 km 2 .Multi-phase tectonic uplifts establish the basin's current limits, and the current basin geometry shows little resemblance to the much larger North African sedimentary basin that existed during the early Paleozoic.The basin has several generations of fault movement; however, the resulting degree of deformation is minimal.The basin has a sedimentary fill that reaches a maximum thickness of about 4000 meters in the basin center and is composed of a large marine Paleozoic portion and a continental Mesozoic layer (Davidson et al., 2000).I-NC186 / R-NC115 fields are located in Murzuq Basin, South West Libya, to the south of A-NC186 Field and to the north of N-NC115 Field (Fig. 1a).Mamuniyat Formation oil is produced by the two fields.Martin (2000); Davidson et al. (2000); Echikh and Sola (2000); Fello (2001); Hallett (2002); El-Ghali (2005); Le Heron et al. (2006); Ramos et al. (2006); Le Heron (2010); Cubitt et al. (2011);Gruber et al. (2011);Castro et al. (2012); Abushalah and Serpa (2016); Bataller et al. (2021).The integrating well logs and core data to improve the reservoir petrophysical characterization were applied by ment authors; for example, Mamaseni et al., 2022).
The current study aims to integrate the geophysical well records and the core analysis to attain the optimal use of the available data to evaluate the Mamuniyat reservoir.In spite of significant exploration success, more than 20 barren holes have been drilled over the past decade (Ron Mart'in et al., 2016), highlighting the significance of the present study.

Geologic Setting
The geologic map of Murzuq Field is illustrated in Fig. 1a.The I&R fields are an anticline structure sitting at the basement of a tilted block that is limited by (to the east) a major NW-SE trending fault (fault sealing has not yet been confirmed) and (to the north -northwest) the field is limited due to reservoir property degradation, as demonstrated by wells I-03 and B1-68.To the west, I&R field structures are dropping below the oil water contact (OWC) at -3142 ft.TVDSS, as shown plainly at well R03-NC115.The I&R fields are stratigraphic and structural trap combined.Oil buildup took place in one of the glacial paleo-valleys occupied by the Mamuniyat Formation, which was eroded and capped by significant Silurian marine transgression (forming the Taconic Unconformity) and gently distorted throughout the basin's subsequent orogenic stages (Castro et al., 2012).During the Silurian Period, the main regional seal is thick shale (Tanezzuft Formation).The principal source rock in the area is the Hot Shale member.The producing reservoir, like the key oil fields A, B, H, M-NC115, and B-NC186, is the Upper Ordovician Mamuniyat Formation (Castro et al., 2012).Within an active glaciotectonic environment, the Mamuniyat Formation is divided into many depositional stages, making the use of traditional sequence stratigraphy, meant for passive margins, challenging.
McDougall and Martin (2000) classified Mamuniyat into three parts: Lower, Middle, and Upper Mamuniyat.In general, three distinct depositional habitats have been identified: Lower Mamuniyat is a high stand systems tract related to the Melaz Shuqran Cycle and sharing debris flow and soft sediment deformation characteristics.The Middle Mamuniyat is shortened to the Lower Mamuniyat, which is composed of rippled, dewatered, and slumping heterolithic.Overall deposition took place in a rapidly progressing braid-delta system fueled by melting ice sheets.Upper Mamuniyat has the best reservoirs properties, which are composed of coarse to extremely coarse-grained deposits built as anastamosed to braided systems on active glacial outwash plains or sandur.
Mamuniyat Formation (also known as Clean Mamuniyat) on I&R fields corresponds to the Lower Mamuniyat sequence in the I-NC186 Field and the eastern half of the R-NC115 Field region.Upper Mamuniyat may occur in the center and western parts of the R-NC115 area, but this must be validated.This pattern might represent a cut and fill cycle, with the basal incision induced by glacial advance and a dip in relative sea level rise.Lower Mamuniyat series with sand-shale intercalation correlates to Dirty Mamuniyat.Middle Mamuniyat was most likely eroded over I&R fields because of its thick shale intercalation with silty-sandstone, which was well maintained in field B-NC115 (high productive similar reservoir on Murzuq basin).The south-to-north well correlation (R02-NC115 to I03-NC186) indicates that the reservoir quality of Clean Mamuniyat declines towards the north and northwest.Due of weak reservoir qualities at the well, the current Clean Mamuniyat reservoir is not regarded to exist at wells I03-NC186 and B1-68.During testing, the I03-NC186 well produced no oil at the top sand bodies, and traditional core analysis at this interval reveals very poor permeability values.Until additional data is available, Well B1-68 is assumed to be at the boundary of the Clean Mamuniyat reservoir interval (McDougall and Martin, 2000).

Materials and Methods
Eleven boreholes, Figure 1b, with complete package of geophysical well records (including gamma ray, deep, shallow, and micro resistivity, neutron, sonic, and density logs) were provided to accomplish the objectives of the present work.Four wells at I-NC186 Field; I-1, I-2, I-10, and I-14 were obtained along with seven wells at R-NC115 Field; R-1, R-2, R-6, R-7, R-9, R-10, and R-14.Besides the given well logs, conventional core data from I-14 Well was also provided to integrate the core and log data for better reservoir characterization of the investigated formation, Mamuniyat Formation.Ip2018 software helped interpret the well logs and core data and the Microsoft Excel software helped create the core-log porosity correlation chart (Al-Ateya et al., 2022).The current study started with preparing the data, loading logs, applying environmental corrections, interpreting the data, and finally presenting the results.The evaluation of the Mamuniyat Formation passed through the following stages: starting with identifying the reservoir level and its mineralogical composition through both log responses and lithological identification cross plots (including MN, neutron-density, neutron-sonic, and sonic-density).The following step was calculating the reservoir's shale content using two methods: single clay indicator using gamma ray log, and double clay indicator using neutron and density logs.After that, the calculated shale volume was used as essential input with resistivity, neutron and density logs to estimate total and effective porosity.Pickett's plot was used to calculate the formation water resistivity (Rw) that was used in calculating water and hydrocarbon saturation.The provided core data and the log-derived results were integrated to achieve a better reservoir characterization in the study area.The surface spectral gamma ray or core gamma ray was used to shift the depth of the logs.Core data analysis was also used to estimate the grain density that was used in calculating the total and effective porosity.

Results and Discussion
Using the responses of different well logs such as gamma ray, density, neutron, and resistivity logs and using many lithological identification cross plots the Mamuniyat Formation was allocated and its lithological content was investigated.MN cross plot (Fig. 2a) shows that the Mamuniyat Formation is made up of sandstone as almost all the points are located in the quartz region.The lithological content of the Mamuniyat Formation was confirmed through other cross plots including neutron-sonic, neutrondensity, and sonic-density cross plots (Figs. 2b,2c,and 2d respectively).
Those cross plots show that sandstone is the dominant component in the Mamuniyat Formation in that field.The thin section obtained from the cored interval of the Mamuniyat Formation (Fig. 3) contains mainly quartz and some clays beside the pores confirming the lithological composition derived from the previously investigated cross plots (Fig. 2).The Mamuniyat Formation has very low shale content that ranges between 5 and 10 %.It also has moderate effective porosity values that vary from 13 to 16% and high hydrocarbon saturation values between 62 and 91%.Table 1 reveals the average values of the calculated petrophysical parameters of the Mamuniyat Formation.The vertical and lateral distributions of the log-derived petrophysical parameters are presented through the lithosaturation cross plots (Fig. 4) and isoperimetric contour maps of the Net pay, Shale volume, effective porosity, and hydrocarbon saturation (Figs. 5a,5b,5c,and 5d), respectively.The spectral gamma ray of the core sample (Fig. 6) shows the presence of uranium, thorium, and potassium in the cored interval of the Mamuniyat Formation.The grain density value of the cored interval was estimated through plotting the measured grain density values obtained from the core data analysis.Grain density value of 2.65 g/cc was chosen to be the value that represents the grain density of the sandstone levels of the Mamuniyat Formation.Fig. 7 illustrates the frequency and cumulative frequency percent of the grain density values revealing the median, mode and arithmetic mean of the grain density values.The mode, 2.65 gm/cc, was chosen to represent the grain density of the formation and was used in the porosity calculations.The correlation between the log-derived and log-derived effective porosity shows a very high correlation coefficient of R2 equalls 0.99 (Fig. 8).Using the core-derived garin density in calculating the effective porosity from neutron and density logs, helped reach a very accurate porosity values.

Conclusions
The present work integrating all available data including well logs and core data analysis to evaluate the Mamuniyat Formation reservoir and investigate the issue of dry holes in Murzuq Basin.According to the integration between the responses of different well logs, lithological identification charts, and the thin section obtained from the core sample, the lithology of the Mamuniyat Formation was identified as sandstone.The shale content was calculated via well logs and confirmed through the spectral gamma carried out on the core sample.It varies between 5 and 10 % .The core data was used to estimate the Grain Density.The core-derived grain density was utilized to estimate the effective porosity from the porosity logs including density, neutron, and density logs.The estimated effective porosity ranges between 13 and 16%.A good match was observed between the core porosity and log porosity with correlation coefficient equals 0.99.The oil saturation varies from 62% to 91%.In general, the Mamuniyat Formation has good reservoir properties, and the increasing number of dry holes could be attributed to other factors other than the reservoir quality.

Fig. 1 .
Fig. 1.(a) Geological map of the Murzuq Basin, modified after (Shalbak, 2015; Gil-ortiz et al., 2022) (b): a base map showing the location of the studied boreholes in the area

Fig. 4 .
Fig. 4. The lithosaturation panel illustrating the vertical distribution of the calculated petrophysical parameters of, left; R-01 Well, right I-01 Well

Fig. 5 .
Fig. 5.The lateral distribution of the estimated petrophysical parameters through the contour maps of (a) Net pay, (b) Shale volume, (c) Effective porosity, and (d) Oil saturation