Burial evolution of sedimentary successions in the Lurestan Basin, NW of Zagros Folded Belt, Iran

Abstract The main objective of this thesis is to unravel burial evolution and thermal-maturation history of the Mesozoic-Cenozoic sedimentary successions of the Lurestan basin in NW Zagros, Iran by integrating petrographic (e.g.vitrinite reflectance) thermal parameters and inorganic (e.g. illite content in mixed layers I-S) thermal parameters derived by the optical analysis of the organic matter dispersed in sediments and by XRD diffraction analysis of clay minerals. The integration of vitrinite reflectance and illite content in I-S may provide important pieces of information on heating rates that cannot be provided by a single indicator. Vitrinite can be used to estimate the degree of maturation of organic matter at low to high temperatures, even to the graphite-grade metamorphic facies. By the1D modeling provided, maximum temperature experienced by the Lurestan sedimentary and the amount of eroded material through time will be determined. Then thermal maturity and quality of source rocks providing insights for hydrocarbon generation and exploration will be determined by Rock-Eval pyrolysis results. The samples collected from eight wells and two sections. Results obtained from organic and inorganic laboratory analysis have been described in the chapter 4 and then integrated for making 1D modeling by Basin Mod1 software and discussed in chapters 5 and 6. Data obtained in this Ph.D. work, in fact, were used to characterize and discriminate the thermal evolution of the Mesozoic and Cenozoic sedimentary succession by focusing more on the Garau Formation as a main source rock in the Lurestan basin. 1D modeling indicate that illite content in I-S is a better paleothermal indicator than vitrinite reflectance in the Lurestan basin. Vitrinite reflectance showed a complex distribution of organic macerals with a wide range of values in most of the wells without a clear trend as a function of depth. In addition, organic thermal indicators often point to higher levels of thermal maturity than those recorded by clay minerals. This could be due to some reasons as following: High vitrinte reflectance values are generally due to dishomogeneity of the organic matter surface, proximity to bright components such as pyrite, size of the particles, orientation of vitrinite fragments due to surface relief/irregularities, mixture of fragments (suppressed/reworked) with the indigenous population of vitrinites, and surfaces with imperfections or "noise" - scratches, (Borrego et al., 2006). Vitrinite macerals can also have different hydrogen contents that may affect thermal alteration. Hydrogen-poor and oxygen-rich vitrinites show higher levels of thermal maturity than those recorded hydrogen rich-vitrinite fragments. Also, vitrinite macerals can be reworked or organic matter can be used as additive during drilling procedure (NIOC personal communication) clouding the thermal signal due to burial. Burial histories calibrated by inorganic thermal parameters revealed a decrease in levels of thermal maturity form the internal to the external part of the Zagros belt. In particular, in the internal part of the belt the Garau Fm experienced maximum sedimentary burials in the order between 3.95 km and 5.45 km and maximum temperatures between 124 and 169°C whereas lower burial conditions occurred in the external part (between 3.65 and 4.9 km) with associated temperatures in the range of 114 - 156°C. These values directly come from the 1D modeling. The onset of hydrocarbon generation for the Garau Fm. occurred before the folding stage of the Zagros belt, from Maastrichtian to early Eocene time. The thickness of eroded rock units decreases toward the external sectors as well from 1.8 - 3km to 1.2 - 2.4 km. Exhumation took place in late Miocene in the internal part of the belt and in early Pliocene time in the external part according to the age of growth strata in the Agha jari and Bakthiari Fms. The base age of the Agha jari Fm is Upper Miocene. The thickness of the Garau Formation in internal part ranges between 577 and 799 m and in external ranges between 200 and 1000 m. The bigger thickness of the Garau Formation is in Kabir Kuh anticline. The Garau Formation by the maximum burial indicate middle mature stage (the Huleylan #1, Mahi Dasht #1, Bankul #1, Samand #1 wells, and Kabir Kuh section), late mature (Baba Ghir #1, Darreh Baneh East #1 wells, and Tang-e Haft section) and overmature stages of hydrocarbon generation and main gas zone (North Shah Abad #1 well). The onset of hydrocarbon generation in the North Shah Abad #1, Baba Ghir #1, Samand #1 wells, and Kabir Kuh and Tang-e Haft sections occurred in the Late Cretaceous time (pre-folding stage), earlier than the other wells, and in the Bankul #1, Mah Dasht #1, Darreh Baneh East wells occurred in the Early Eocene time (pre-folding stage). The onset of hydrocarbon generation in the Huleylan well occurred later than the other wells in the Paleocene time occording the modeling. Vitrinite reflectance data present the late mature and overmature stage of hydrocarbon generation for the wells and sections in internal and external part except Huleylan #1 well that vitrinite reflectance value ranging between 0.61% -0.89% that indicate the peak maturity level for producing of oil. Clay mineralogy data define their paleo-temperatures as they provide a signal throughout the entire sedimentary succession. Respect to Merriman and Frey’ chart ( figure 3.2.1.1), all the samples belong to internal and external part of the Lurestan basin have a potential of producing of hydrocarbon except Darreh Baneh East #1 by random structure Ro and I% in Mixed layers I-S between 46 and 65%. The HI versus Tmax graphs also indicate that the sediments are of type III kerogen and gas prone and they are mature (early - late) and overmature. The TOC contents in all samples of the Garau Formation show a wide range of hydrocarbon potential from poor to excellent. The plot of S1 vs.TOC and show the presence of indigenous hydrocarbon in all samples.