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Integrated Geologic, Geophysical, and Petrophysical Data to Construct Full Field Geologic Model of Cambrian-Ordovician and Upper

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Integrated Geologic, Geophysical, and Petrophysical Data to Construct Full Field Geologic Model of Cambrian-Ordovician and Upper t Technical papers Integrated geologic, geophysical, and petrophysical data to construct full field geologic model of Cambrian-Ordovician and Upper Cretaceous reservoir formations, Central Western Sirte Basin, Libya Abdalla A. Abdelnabi1, Yousf Abushalah2, Kelly H. Liu1, and Stephen S. Gao1 Abstract The Cambrian-Ordovician and Upper Cretaceous formations, which are the main oil-producing formations in the central Sirte Basin, are structurally complex. The lateral and vertical heterogeneity of the reservoir forma- tions is not well-understood, which negatively affects the performance of the reservoirs. We constructed effi- cient full-field static models that incorporate the lateral and vertical variation of those reservoir formations by integrating geologic and geophysical data. We determined lithology and reservoir properties by selecting ap- propriate petrophysical techniques that suit the available well data and overcome issues with unreliable well-log measurements. In the process of building structural models, defining and mapping the base of the Cambrian- Ordovician Gargaf Formation was very challenging because wells did not penetrate the basal formation, and the quality of the seismic data decreases with depth. Therefore, we applied techniques of adding isochore maps of the overlying Upper Cretaceous of the Bahi and Waha Formations to map basal contact and determine the thickness of the Gargaf Formation for the first time in the area. The constructed isochore maps showed the thickness variation and the distributions of the Bahi and Waha Formations and explained the influence of Gargaf paleotopography and faults on them. The fault models combined with facies and property models suggested an interconnection among the three main reservoirs. They also indicated that the quality of the Waha reservoir enhances as the lithology varies from limestones to calcareous sandstones, whereas the quality of the Gargaf reservoir was primarily controlled by fractures. The total estimate of the original oil in place with the largest contribution of hydrocarbon volume from the Waha Formation was 2.55 × 109 stock tank barrel. The created 2 model with a fine-scale geocellular covering an area of 350 km is unique to the study area and it can be updated and refined at any time with new data production and drilling activities. Introduction vician Gargaf, Upper Cretaceous Bahi, and Waha with a The Sirte Basin is among the world’s top 20 petroleum large variability in thickness and lithology in the study provinces, and it is one of the four known sedimentary area (Figure 2; Cain, 2001). In spite of numerous studies, basins in Libya (Figure 1a). It covers a total area of no consensus has been reached on the degree of connec- 600;000 km2 and is the richest and youngest sedimentary tion among the different layers of the reservoirs and on basin in the country, with known reserves of more than the factors that distinguish these formations. As a result 43.1 billion barrels (Barr, 1972; Futyan and Jawzi, 1996; of its limited spatial distribution and thickness, the Bahi Wennekers et al., 1996; Coward and Ries, 2003; Craig Formation contains the least hydrocarbon reserves. How- et al., 2004; Abadi et al., 2008). The major structure of ever, it is of significance because it acts as a good vertical the Sirte Basin was developed between the Late Paleo- and lateral reservoir conduit between the Waha and zoic and the Early Cretaceous, during which a continuous Gargaf reservoir units wherever it is present. Previous uplift formed the Sirte Arch. During the Late Cretaceous, studies suggested that it is problematic to differentiate extensional tectonics resulted in a partial collapse of the Bahi detritus and quartzitic materials from similar depos- Sirte Arch and caused a system of horsts and grabens its found in either the Waha or Gargaf units (De Golyer (Figure 1b; Klitzsch, 1971; Bellini and Massa, 1980; Van and Macnaughton, 2010). Houten, 1983; Ahlbrandt, 2001; Hallett, 2002). There are Integrating geologic, geophysical, and petrophysical three primary reservoir formations: the Cambrian-Ordo- data into one complete model of the whole field is an Downloaded 12/13/18 to 131.151.109.237. Redistribution subject SEG license or copyright; see Terms of Use at http://library.seg.org/ 1Missouri University of Science and Technology, Geology and Geophysics Program, Rolla, Missouri, USA. E-mail: [email protected]; liukh@ mst.edu; [email protected]. 2University of Texas at El Paso, Geological Sciences, El Paso, Texas, USA. E-mail: [email protected]. Manuscript received by the Editor 22 December 2017; revised manuscript received 17 September 2018; published ahead of production 12 October 2018; published online 10 December 2018. This paper appears in Interpretation, Vol. 7, No. 1 (February 2019); p. T21–T37, 17 FIGS., 3 TABLES. http://dx.doi.org/10.1190/INT-2017-0236.1. © 2019 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved. Interpretation / February 2019 T21 important process to help better understand the com- data conditioning to illuminate some of the unrevealed plexity of reservoir formations, thereby facilitating sig- geologic features (Hamarbatan et al., 2006; Abushalah nificant gains in productivity in terms of identifying and and Serpa, 2016, 2018; Abdelnabi et al., 2017). In a pre- managing reservoirs (Zou et al., 2006; Denney, 2013; vious study (Abdelnabi et al., 2017), the authors used Adeoti et al., 2014). It provides reservoir engineers with frequency enhancement and curvature attributes to iden- a reliable and comprehensive basis upon which they tify minor faults in the study area and the distribution of can simulate and manage reservoirs (Al-Harbi et al., the fractures in the Cambrian-Ordovician reservoirs. 2013; Karamitopoulos et al., 2013). In general, however, Here, we present a study of such an integrated approach combining these various forms of data represents a sig- usingacombineddatasetfromtheRG-oilFieldinthe nificant challenge, mostly because of the fact that reliable Sirte Basin in Libya. This case study uses a method by integration requires a comprehensive understanding of which a comprehensive blend of geologic, geophysical, the inherent uncertainties of the numerous components petrophysical, reservoir engineering information, and the in a given geologic system (Hosseini et al., 2012; Kamali accompanying interpretations are linked in one static et al., 2013; Musa et al., 2013; Rodríguez Torrado et al., model. The modeling approach presented here is unique 2015). Because the availability and variability of well data to the study area. One of the aims of this study is to char- that were acquired by different companies in different acterize the heterogeneity of the main producing forma- years and quality of seismic data, it was required to use tions in the study area. The workflow of the construction certain analysis techniques and data enhancement to of the full field static reservoir model of the RG-oil Field is overcome data limitations. Applications of frequency en- illustrated in Figure 3. hancement and filtering of seismic data on some oil fields in Libya, for example, demonstrated the importance of Geologic background and database Regional geologic setting and petroleum system The RG-oil Field is located in the west-central part of the Sirte Basin (Fig- ure 1). The major structural features that have affected sedimentation in this area include four northwest–southeast- trending structures (Van Houten, 1980; Anketell, 1996; Schroter, 1996), which are the Zallah and Tumayan Troughs to the west, the Beyda Platform in the cen- tral part of the area, and the Maradah Graben to the east (Parsons et al., 1980; Mansour and Magairhy, 1996; Capitanio et al., 2009). All platforms in the region are covered by limestone and remained relatively stable throughout the Early Paleocene. However, the shale-filled troughs continued to subside and deepen to the north particularly along the Ajda- biya trough to the present day (Gumati and William, 1985; Baric et al., 1996; Bel- haj, 1996; Heselden et al., 1996; Johnson and Nicoud, 1996). The stratigraphy of the Sirte Basin was divided into four megacycles: Paleozoic, Triassic to Early Cretaceous, Late Cretaceous to early Oligocene, and Oligocene (Montgomery, 1994). The cycles characterize stages of pre-, syn-, and postrift sequences. The Cambro-Ordovician nonmarine clastics from the oldest megacycle (a prerift se- quence) are represented by the Gargaf/ Downloaded 12/13/18 to 131.151.109.237. Redistribution subject SEG license or copyright; see Terms of Use at http://library.seg.org/ Hofra sandstone, which is underlain by Figure 1. (a) Location map of the study area as indicated by the red rectangle in the Sirte Basin, Libya. (b) A structural map showing the main structural features a Precambrian unconformity and over- affecting the sedimentation pattern in the study area. The blue areas represent laid by late Permian or Triassic uncon- major highs, the red areas represent gas field distribution, and the green areas formity. The sequence is characterized represent oil fields (modified after Rusk, 2001). by tightly cemented sedimentary quartz- T22 Interpretation / February 2019 ite, which is enhanced by fractures to form a secondary throughout the basin (Gumati and Nairn, 1991; Roohi, reservoir in the study area (Ahlbrandt, 2001). The second 1996a, 1996b; Sinha and Mriheel, 1996). The reservoirs cycle is dominated by alluvial deposits of the Nubian For-
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