Deciphering the Reservoir Rocks Lithology by Mineralogical Investigations Techniques for an Oilfield in South-West Romania

. An important element of the geological modeling of oil reservoirs is represented by determining of the mineralogical composition and rock types as part of the reservoir characterization process. In the paper we provide a comprehensive mineralogo-petrographic study based on petrographic observations and X-rays diffraction investigations made on several Miocene rock samples collected in the wells spudded in an oil field belonging to the Getic Basin. Getic Basin is a prolific petroleum province in Romania and belongs to petroleum systems of the Carpathian Foredeep. The oil exploration in the Getic Basin started more than 100 years ago and resulted in thousands of wells drilled and tens of fields discovered. The oil field is located in the Gorj County, geologically belongs to the internal zone of the Getic Basin, and is a faulted anticline with hydrocarbon accumulations in Burdigalian and Sarmatian deposits. The petrographic study led to the interpreting of the rock samples analyzed as epiclastic sedimentary rocks represented by conglomerates, breccias, sands, sandstones, claystones and marlstones, and carbonate rocks (limestones). X-rays diffraction investigations indicated the phyllosilicates (smectite and illite) as main minerals in the Sarmatian samples, while in the Burdigalian samples were found as main minerals: quartz, feldspars and carbonate minerals. The paper provides detailed information (like petrographic types, composition and microtexture) on the Miocene reservoir rocks belonging to the Getic Basin. Also the data obtained may be used as basis for future reservoir modeling studies in the region.


Introduction
Geological modeling of hydrocarbon reservoirs consists of several processes of which it stands out the geological characterization of the reservoirs. The geological characterization process of hydrocarbon reservoirs is a process in which an interdisciplinary scientific model is created that integrates and reconciles different types of geological and hydrodynamic information from the pore scale where petroleum is reservoired to the sedimentary basin scale. As important components of the geological characterization process we mention identification of rock types and establishing of their mineralogical composition. The paper The oilfield studied is located in the Gorj County, 10 km south to the Targu Jiu city, and geologically belongs to the Western Getic Basin and to Calnic-Ciuperceni-Targu Jiu-Colibasi-Alunu structural trend respectively. The oilfield structure is a faulted anticline affected by a tectonic system of longitudinal and transversal faults. The wells spudded in the structure were crossed deposits belonging to Oligocene, Burdigalian, Sarmatian, and Pliocene geological ages. [9][10]  The exploration phase started in 1951-1957 period and since 1959 the exploration wells were evidenced in the production tests the existence of oil and gas accumulations in Oligocene and Miocene (Burdigalian and Sarmatian) deposits and confirmed the commercial oil discovery. In the oilfield studied were drilled over 50 wells of which 26 wells have positive results in production tests and 21 wells entering in production, and daily production (Burdigalian and Sarmatian reservoirs) is around 3,000 barrels per day. [9][10] The average values of main parameters of the oil-bearing formations in the oilfield studied are presented in the table 1.

Experimental data
In order to perform the petrographic study were manufactured thin sections from the rock samples belonging to Burdigalian (classical Helvetian) and Sarmatian. Thin sections were prepared to investigate the optical properties of the minerals in transmitted light and helping to reveal the rock types, origin and evolution of the parent rock. The petrographic study of the thin sections is essential to correct interpretation of rock types and their petrogenesis. The petrographic study was performed using a polarizing optical microscope Leica type. Pictures were taken with a capture incorporated camera.
The mineralogical composition of the rock samples analyzed was determined by X-rays diffraction (XRD) technique. In order to prepare the powders for the measurements the rock samples were grinded in an agate mortar. XRD data were obtained using a Bruker D8 Advance diffractometer under following measurement conditions: CuKα radiation (λ=1,54Å; 40kV; 40mA), Bragg-Brentano geometry, step 0.1º, scan speed 0.1º/5s, and measurement range (2θ) 1-60º. In order to modeling of the emission source profile were used NIST SRM 660a and SRM 676 profile standards. XRD measurements were interpreted both qualitative and quantitative. Diffracplus Basic software and PDF-ICDD 2-2008 database were used for qualitative interpretation of XRD measurements, while TOPAS 4.1 software was used for quantitative interpretation of XRD measurements by Rietveld method (refinement). The figures of merit of the Rietveld refinements are represented by the Rwp (R-weighted pattern), GOF (goodness-of-fit) and DW (Durbin-Watson d-statistic). [11][12][13]

Results and discussions
Petrographic study of the thin sections led to the conclusion that rock samples analyzed belong to the epiclastic sedimentary rocks (conglomerates, breccias, sands, sandstones, claystones and marlstones), and carbonate rocks (limestones).
Our goal was to provide a detailed characterization of these rock samples in terms of variety, composition and microstructures, information that cannot be obtained by indirect investigation methods such as geophysical and hydrodynamic investigations. The samples analyzed are composed of detrital fraction (represented by granoclasts, lithoclasts and bioclasts) and a fine-grained, clay-carbonate matrix.
The samples belonging to the Sarmatian are characterized by a fine-grained lithofacies (claystones, marlstones and fine sands) while the cores from Burdigalian are characterized by a coarser lithofacies (conglomerates, breccias, sandstones).
The samples of Burdigalian age (i.e. SWC 1 to SWC 20) were assigned to clast-arenites rocks like greywacke, clayey greywacke, clay sands, accidentally detrital limestones, in the base of the Burdigalian deposits, and to the top of Burdigalian sedimentary suite mainly clast-rudites rocks like conglomerates and microbreccias with metamorphic elements (microquartzite, greenschists, gneiss, mylonite, granitoids) and respectively limestones.
The four samples of Sarmatian age (i.e. SWC 21 to SWC 24) revealed a transition to fine-grained (clayey-marly) fraction: in the base marlstones and clayey sands, and to the top calcareous claystones. The microscopic investigations carried out on the sample rocks analyzed are summarized in the table 2.  XRD investigations were performed both qualitative and quantitative. In the XRD qualitative interpretation, the identification of the minerals using PDF-ICDD 2-2008 database was performed searching best quality marks: (*=high quality) and (I=indexed), after removing the background and Kα2 radiation. XRD quantitative interpretation of the samples analyzed was performed by Rietveld method. Quality of the Rietveld refinement is indicated by R-values (GOF and DW). XRD investigations show significant percentages for carbonate minerals (calcite), clay minerals (smectite, illite and clinochlore) and siliciclasts (quartz, plagioclase, microcline, micas). The mineralogical composition of rock samples analyzed resulted in the XRD quantitative analysis (wt% Rietveld) is presented in table 3.

Conclusions
Getic Basin is a major geological unit with a complex tectonic evolution in the Romanian Carpathian Orogen. In the Getic Basin during Tertiary period were acting and evolving simultaneous two petroleum systems. The cores analyzed in the paper were collected in wells spudded in an oil field located in the South-West Romania, and geologically belonging to Western Getic Basin and locally to Calnic-Ciuperceni-Targu Jiu-Colibasi-Alunu structural trend.
In the paper we focused on detailed characterization of the rock samples studied in terms of petrographic types, composition and microtextures, information that cannot be obtained by indirect investigation methods such as geophysical and hydrodynamic investigations.
Petrographic study led to the conclusion that rock samples analyzed belongs to the epiclastic sedimentary rocks represented by conglomerates, breccias, sands, sandstones, claystones and marlstones, and carbonate rocks (limestones). The samples analyzed are composed of detrital fraction (represented by granoclasts, lithoclasts and bioclasts) and a fine-grained, clay-carbonate matrix. The samples belonging to the Sarmatian are characterized by a fine-grained lithofacies (claystones, marlstones and fine sands) while the samples from Burdigalian are characterized by a coarser lithofacies (conglomerates, breccias, sandstones). The samples of Burdigalian age (i.e. SWC 1 to SWC 20) were assigned to clast-arenites rocks like greywacke, clayey greywacke, clay sands, accidentally detrital limestones, in the base of the Burdigalian deposits, and to the top of Burdigalian sedimentary suite mainly clast-rudites rocks like conglomerates and microbreccias with metamorphic elements (microquartzite, greenschists, gneiss, mylonite, granitoids) and respectively limestones. The four samples of Sarmatian age (i.e. SWC 21 to SWC 24) revealed a transition to fine-grained (clayey-marly) fraction: in the base marlstones and clayey sands, and to the top calcareous claystones.
XRD investigations indicated the phyllosilicates (smectite and illite) as main minerals in the Sarmatian samples, while in the Burdigalian samples were found as main minerals: quartz, feldspars and carbonate minerals. The paper provides detailed information (like petrographic types, composition and microtexture) on the Miocene reservoir rocks belonging to the Getic Basin, and the information obtained may be used in future reservoir modeling studies in the region.