Applications of Biological Markers to Characterize Petroleum and Source Rocks of the Southern Indus Basin

Abstract

Crude oils (20) and sediments (435) from the Southern Indus Basin have been geochemically studied. The analytical techniques used in this study are TOC analysis, Rock Eval, and gas chromatography-mass spectrometry. Chapter 1 of this thesis presents an introduction of sedimentary OM, kerogen, aliphatic and aromatic biomarkers 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 Southern Indus Basin. The experimental procedures and techniques for data collection and analysis are explained in Chapter 3. In chapter 4, distribution and relative abundances of saturated and aromatic hydrocarbons have been applied to distinguish different oil families. Biomarker parameters divide the samples into three groups on the basis of source, depositional environment and lithology, thermal maturity of organic matter and in reservoir mixing study. Group I (BN, Palli, Sono, Thora, Dhamrkhi, BJ, T Alam, BMD, and BH) shows high Pr/Ph (3.26-4.46), bimodal n-alkane distribution and high abundance of C 20 + n- alkanes. High concentration of 1,2,5-TMN, 1,7-DMP, high 1-MP/9-MP, presence of retene and low C 27 /C 29 steranes support their likely origin from terrestrial source (coniferous plant). Group II (BS, BA, T A Yar and Resham) has moderate to high Pr/Ph (2.84-3.67), bimodal n-alkane distribution, presence of retene, low 1-MP/9-MP and moderate concentration of 1,2,5-TMN, 1,7-DMP. These samples probably have mixed (terrestrial and marine) source of organic matter. Group III of condensates (Baloch, Unnar, Shah, Pasahki East, Pakhro, Chandio and Gopang) is extremely low in biomarkers. The source rocks, probably marine oxic clastics having terrestrial OM have generated these waxy condensates under high thermal conditions. Marine oxic clastic source rocks for these condensates are also reflected from plots: DBT/P versus Pr/Ph, C 29 H/C 30 H versus C 29 -dia/(dia+Reg) steranes, Pr/Pr+Ph versus C 29 -dia/(dia+Reg) steranes and C 27 /C 29 sterane versus C 27 /C 29 -diaseranes. The maturity parameters, TMNr, TeMNr and PMNr, MPI, R c MDR and R cs show a wide range of thermal maturities. Palli crude oil has the lowest maturity among the sample suit while condensates have highest maturity. A novel maturity parameter 4-MDBT/1,2,3,5,6-PMN has been proposed in this iiichapter which correlates very well with other established maturity parameters. Ternary plot of TMNr, TeMNr and PMNr reflects that majority of the analyzed crude oils are indigenous in nature with the exception of Palli, Pakhro and Sono which may have some in-reservoir mixing. Chapters 5-7 are dedicated to geochemical evaluation of sedimentary sequences: Ranikot (Paleocene); Laki (Eocene); Parh, Goru and Sembar (Cretaceous) and Datta and Chiltan (Jurassic) using Rock Eval and aliphatic and aromatic biomarkers. In chapter 5, aliphatic and aromatic biomarkers along with TOC and Rock Eval study have been used to evaluate geochemistry of sediments from Ranikot (Paleocene) and Laki (Eocene) Formations. Low values of T max , biomarkers and aromatics maturity parameters and high abundance of hopenes and ββ-hopanes support highly immature nature of organic matter in both Laki and Ranikot sediments. Bimodal n-alkane distributions, marked predominance of C 27 , C 29 and C 31 n-alkanes, high abundance of C 29 steranes compared to C 27 and C 28 homologues, presence of oleanene suggest that the organic matter in these sediments is predominantly of terrestrial origin, while cadalene and its intermediate like calamenene and tetrahydrocadalene specify drought resistant low land plants. The occurrence of C 30 steranes and high concentration of fungal derived perylene supports the contribution of fungal organic matter and specific warm and humid climatic conditions supporting low land or marshy conifers on which wood degrading fungi flourish. It is most likely that raise in sea level and increased salinity sea waters may have inundated these low lands and shortage of oxygen and minerals resulted in death and preservation of fungal colonies along with wood debris. Marine anoxic to sub- oxic conditions (Pr/Ph 0.43-2.01) in water columns favored peryloquinones formation and finally conversion to perylene and preservation in Laki and Ranikot Formations in high abundance. A thin section within the Laki Formation (460-490 m) has mixed type II, III kerogen, reflected from HI: 224-540 mgHC/gTOC and Pr/n-C 17 versus Ph/n-C 18 plot. Therefore, despite good to excellent organic richness, these sediments are highly immature and far from hydrocarbons generation. The main objective of chapter 6 is geochemical characterization of Parh, Goru and Sembar Formations sediments using TOC, Rock Eval and gas chromatography-mass ivspectrometry. Most of the sediments of Parh, and Upper Goru Formations are lean in organic matter (TOC < 1 %) and immature (T max 420-423 oC; PI < 0.15) while those of Upper Shale, Lower Shale and Talhar Shale sub-units of Lower Goru Formation are comparatively organic rich (TOC 1-3 %) and mature (T max > 435 °C). The upper 430 m section of Sembar Formation shows high TOC (1-3.2 %) and maturity status from peak oil to overmature (T max : 442-487 oC), while lower 190 m including Jurassic sediments are poor to fair in organic matter (TOC: 0.5-1.5 %), however, T max mostly lies in the mature zone with the exception of some bottom sediments. S 2 /S 3 values in the range of 0.06-9.35 suggest gas generation potential of Sembar Formation. Modified Van-Krevlen diagram and plot of T max versus TOC suggest type III kerogen in the Lower Goru and Sembar shales. Biomarker data support Rock Eval results. Molecular maturity parameters like MPI, R c , MDR, R cs , TMNr and TeMNr and plots of 20S/(20S+20R) versus ββ/(ββ+αα) for C 29 steranes indicate low maturity of Parh, Upper Goru, Upper Shale and Lower section of Sembar Formation and medium to high maturity of Lower Shale, Talhar Shale and most of the samples of Sembar Formation. n-Alkanes distributions, sterane ternary plot, presence of retene, high abundance of 1,2,5-TMN and 1,2,5,6-TeMN suggest predominant contribution from terrestrial organic matter in Parh, Upper and Lower Goru Formations and mixed marine and terrestrial organic matter in Sembar Formation. Values of Pr/Ph, C 35 /C 34 , C 31 R/C 30 , C 29 /C 30 hopanes suggest marine sub-oxic to oxic depositional environment for Parh, Upper Goru and Sembar Formations and oxic to sub-oxic nearshore marine or shallow marine environment for the Lower Goru Formation. Chapter 7 is dedicated to characterization of previously unexplored Jurassic sediments of Datta and Chiltan Formations. Maturity parameters, 20S/(20S+20R), ββ/( ββ+αα), 22S/(22S+22R), moretane/hopane, T s /T s +T m , MPR-1, MPR-3, MPI, PhDBF, TrPr-1 and TrPr-2 reveal immature to early maturity status of Datta and comparatively higher maturity level of the Chiltan sediments. Palynological data by Ministry of Petroleum Pakistan on BB well show the presence of Cheirolepidiacean spore classes Classopollis torosus and Spheripollenite along with Araucariacites australis in Datta Formation which are important palynomorphs of conifers. Since conifers appeared in the fossil record of the Permian; it indicates coniferous source and Permian or younger age of sequence. A notable feature of both Datta and Chiltan sediments is the presence of vrelatively high proportions of alkylcyclohexanes and alkylcyclopentanes series. As these compounds are characteristic of marine organic sources, they provide an indication for the marine algal incursion for these Jurassic sediments. The major contribution of organic matter in Chiltan Formation is from terrestrial source shown by high abundance of C 27 , C 29 , C 31 n-alkanes and C 29 steranes and low sterane/hopane ratio, with some contribution from marine algal source as shown above. Datta Formation has mixed marine algal and terrestrial organic matter on the basis of high abundance of C 16 , C 18 , C 20 n-alkanes, presence of C 30 steranes, ratios of sterane/hopanes, tricyclicterpanes/hopanes and high abundances of alkylcyclohexanes and alkylcyclopentanes. The presence of coniferous spores and woody tissues in Datta Formation also support terrestrial organic matter input. Paleoclimatic interpretation of Datta Formation has been made on the basis of high abundance of combustion derived polycyclic aromatic hydrocarbons as a result of wild forest fires during Jurassic times. Classopollis torosus along with Spheripollenite which are associated with warm and humid climatic conditions may have initiated the wildfires and the subsequent deposition of pyrolytic aromatic hydrocarbons in these sediments. Presence of vitrinite and phenyl substituted aromatics like phenylnaphthalenes, phenyldibenzofurans and benzonaphthofurans also supports wild forest fires during deposition of Datta Formation. In chapter 8 diamondoid hydrocarbons have been proposed as maturity indicators of condensates. Both waxy and non-waxy condensates originated from terrestrial organic matter deposited under marine oxic conditions have been examined. High level of thermal maturity is indicated by the values of diamondoid based maturity parameters, like methyladamantane index ranging from 54.1-75.8 % and methyldiamantane index ranging from 34.9-56.3 % which corresponds to vitrinite reflectance 1.1-1.6 %. Methyladamantanes/adamantane 3.99-5.52 and methyldiamantanes/diamantane 2.16- 2.99 indicate non-biodegradation nature of these samples. The data are supported by absence of unresolved complex mixture (UCM).