GEOCHEMICAL SEGREGATION OF PETROLEUM SYSTEMS OF POTWAR BASIN USING GC-MS AND PYROLYSIS TECHNIQUES

Abstract

This study deals with characterization of Petroleum System of the Potwar Basin, Pakistan. For this purpose, crude oils/condensates (12) obtained from reservoirs of Eocene, Paleocene, Jurassic Permian ages, and sediments (121) selected from seven geological formations and six wells namely A, B, C, D, E and F, were geochemically analyzed. The geological formations are: Chorgali and Sakesar (Eocene), Patala, Dhak pass and Lockhart (Paleocene), Datta (Jurassic) and Sardhai (Early Permian). Various methods and analytical techniques were used in this study including TOC, Rock Eval pyrolysis, GC-FID, Gas Chromatography-Mass Spectrometry (GC-MS), Elemental Analysis, Stable Carbon and Nitrogen Isotopes, Spontaneous Potential (SP) log and Gamma Ray (GR) log. Both biomarkers and non-biomarker parameters were applied for characterization of samples. This thesis is comprised of eight chapters. Chapter 1 presents an introduction to: petroleum systems of the Potwar basin, and introduction of analytical techniques and their applications in geochemical evaluation. Chapter 2 describes samples and background geology of the study area, lithological description, petroleum systems and source rocks of the Potwar Basin. The experimental procedures and techniques for data collection and analysis are explained in Chapter 3. Chapters 4 to 8 independently contain abstract, introduction, results & discussion and conclusions. In chapter 4, Spontaneous Potential (SP) and Gamma ray (GR) logs have been used for the identification of productive zones within the sedimentary sequences of Eocene (Chorgali and Sakesar) and Paleocene (Patala) ages. The study encompasses three wells i.e. D, E & F. The order of permeability (reservoir property) from SP log is Chorgali > Sakesar > Patala. Shale contents and organic matter increases with depth within the sedimentary sequences. Patala Formation on the basis of high shale content and organic matter was assigned as potential source rock. Chapter 5 elaborates the hydrocarbon source rock potential of Eocene, Paleocene and Jurassic sediments obtained from three producing wells referred to as Well-A, Well-B and Well- C, using Rock-Eval pyrolysis and total organic carbon (TOC) measurement. In Well-A, the viii

upper ca. 100 m of the Eocene Sakesar Formation contained abundant Type III gas-prone organic matter (OM) and the interval appeared to be within the hydrocarbon generation window. The underlying part of the Sakesar Formation contained mostly weathered and immature OM with little hydrocarbon potential. The Sakesar Formation passes down into the Paleocene Patala Formation. T max was variable because of facies variations which were also reflected in variations in hydrogen index (HI), TOC and S2/S3 values. In Well-A, the middle portion of the Patala Formation had sufficient maturity (T max 430 to 444°C) and organic richness to act as a minor source for gas. The underlying Lockhart Formation in general contained little OM, although basal sediments showed a major contribution of Type II/III OM and were sufficiently mature for hydrocarbon generation. In Well-B, rocks in the upper 120 m of the Paleocene Patala Formation contained little OM. However, some Type II/III OM was present at the base of the formation, although these sediments were not sufficiently mature for oil generation. The Dhak Pass Formation was in general thermally immature and contained minor amounts of gas-prone OM. In Well-C, the Jurassic Datta Formation contained oil-prone OM. T max data indicated that the formation was marginally mature despite sample depths of > 5000 m. The lack of increase in T max with depth was attributed to low heat flows during burial. However, burial to depths of more than 5000 m resulted in the generation of moderate quantities of oil from this formation. In Chapter 6, elemental and isotopic composition of C and N has been applied to interpret the depositional environment of source rocks and relative contribution of marine and terrigenous OM. The study was conducted on 50 sediments analyzed in Chapter 4. High values total carbon contents (TCC) and extremely low total nitrogen contents (TNC) reflect an enhanced amount of terrestrial OM in these sediments. Low values of Pr/Ph (<1) and diasteranes/steranes (~0.2) and high TOC suggest anoxic environments and marine carbonate depositional setting for OM. Carbon isotope ratios of OM generally range from –25.8 to –24.2‰ with lower values occurring in the some samples of Sakesar formation. The values are 2.8‰ greater than −27‰, the mean value of C 3 plants and suggest that OM was derived from C 3 plants with significant input from land plants and marine planktons. The plot of C/N vs. δ 13 C demonstrates that OM in Chorgali and Sakesar samples is from a similar source such as vascular C 3 plant as primary producers. The trend toward low C/N values within the Chorgali and Sakesar formations is associated with ix

inclusion of marine planktonic OM into the source. Similarly low C/N values (< 20) observed for Patala and Sardhai samples imply significant carbon input from marine planktons in mixed OM. δ 15 N data show two trends, low values in the range of 2.3 to 3.8‰ observed for Chorgali, Sakesar and some Patala sediments indicate mixed land plant and marine planktonic OM, while slightly higher values 3.1 to 5.9‰ for Sardhai and Patala (Well-F) Formations illustrate comparatively higher proportion of planktonic input. The δ 15 N versus δ 13 C diagram clearly demonstrates the nature and origin of OM. It is composed of land plants mainly derived from C3 plants having variable proportions of marine planktonic input. In Chapter 7, biomarker, Rock-Eval, TOC data has been used to characterize the OM quantity and quality, and to interpret the depositional environment and thermal maturity in sedimentary sequences of Eocene (Chorgali & Sakesar), Paleocene (Patala) and Early Permian (Sardhai) ages described under chapters 4 & 6. Rock-Eval pyrolysis data indicate that Chorgali and Sakesar Formations have good to very good quantity of type II/III OM with potential mainly for gas generation. The samples have Hydrogen Index (HI) 275-374 mgHC/gTOC and S 2 /S 3 mostly 4.5-5.5. Most of the Paleocene sediments show HI values in the range of 300-445 mgHC/gTOC and suggest major contribution of type II kerogen in these samples; S 2 /S 3 ratios in the range of 5.5-16 indicate both oil and gas prone sediments, while lower values (< 5) reflect gas prone OM. The Early Permian, Sardhai samples have HI 218-354 mgHC/gTOC and S 2 /S 3 up to 6.8 and represent mostly gas prone type II/III OM. All the samples show TOC about 2- 3.6% and T max 440 – 442°C which is consistent with good to very good organic richness and thermal maturity of sediments in the peak oil window. The relative distributions of C 27 –C 29 steranes in order of C 27 >C 29 >C 28 , and C 27 /C 29 steranes >1 suggest OM input of mixed nature, most likely of marine planktonic and terrestrial origin. Low values of diasterane/sterane and Ts/ (Ts+Tm) for most samples (0.2-0.4 and 0.5-0.6) as well as Pr/Ph ratios up to 0.2-0.8 suggest anoxic clay-poor/carbonates having high pH and low Eh. The values of maturity parameters, ββ/ (ββ+αα) and 20S/ (20S+20R) C 29 sterane, are lower than the equilibrium values and represent early generation stage of samples; however, keeping in view T max values 440 – 442°C, and that sediments under study are anoxic carbonates, wherein generation stage is reached before the equilibrium, we propose that all samples have reached the x

peak of the oil window. The variations in biomarker and Rock-Eval parameters in some samples suggest regional variations of organic facies in their source rocks. In chapter 8, crude oils and condensates (12) have been analyzed for diamondoids and biomarkers. GC and GC-MS parameters reveal that these samples are mature and contained marine and algal/bacterial OM sources from an oxidizing environmental/dysoxic environment. The total methyladamantanes/admantane ratios 4.05 to 15.25 show increasing levels of microbial oxidation. The diamantane/adamantane ratios vary from 1.14 to 3.06 also supports the results. The degree and classification of microbial oxidation was defined by plotting American Petroleum Institute gravity versus diamondoid concentrations. This study demonstrated that biomarkers and diamondoids provide the best means to determine the maturity level of crude oils and condensates.