Tectonic Evolution And Hydrocarbon Exploration Prospects In Central Iran Basin

Central Iran basin is a typical Cenozoic retro-arc basin, in which the target layer Qom Formation is the mixed siliciclastic-pyroclastics -carbonate formation, and a typical fracture and pore reservoir. Because of the complexity property of the basin, reservoir characteristics, and reservoir-forming conditions, hydrocarbon exploration in the basin is of great challenge. Based on the theory of petroleum geology and foreland basin analysis, the present paper has synthetically studied the tectonic evolution and petroleum geologic conditions through surface and underground geological conditions, geology-geophysics-geochemistry from point to area. The characteristics of the structure-fault system, tectonic evolution and trap characteristics are studied by using geophysics. The characteristics of rocks and distribution of favorable sedimentary facies belts are studied by using profiles, cores and cutting. The fractured carbonate reservoir are evaluated by using the experimental analysis of mechanical properties and outcrop,core,imaging logging fracture observation, dual laterolog fracture parameter interpret and fracture void quantitative calculation of C4 sub-member in the Aran anticline. The properties of source rocks, filling period of hydrocarbon are studied by using geochemistry, and then condition and process of hydrocarbon reservoir are resumed, regularities of distribution and controlling factor of reservoir are discussed. Finally, the paper has evaluated synthetically petroleum geologic conditions and pointed out that favorable petroleum accumulation zone and targets in central Iran basin. The following achievements have been achieved.1. Regional tectonic evolution of central Iran basin can be divided into four phases: the Craton depression, the compressive-extensional stage in Mesozoic Era; the retro-arc extensional basin in Paleocene-Eocene, the collision-orogenesis- ??extension- compressive in Oligocene-Quaternary.2. The main structure strike of Kashan area distributed in NW-SE direction. Kashan area can be divided into four first-tectonic units from south to north: foothill thrust-folded belt, foothill depression, central uplift and slope belt. Kashan area underwent compression in late Alpine movement. Tectonization in late Alpine movement can be divided into five strong tectonic episodes based on the characteristics of stratum sediment and unconformity. Tectonization effects on Aran anticline exhibited as the follows: Aran anticline begin form rudiment in Ml member of URF; stronger compressive aggrandized extent of uplift in M2 member of URF; the strongest Tectonization aggrandized ultimately extent, especially in curvature of Aran anticline in later Quaternary sediment, Aran anticline finalized the design.3. Thin section identification results show sedimentary hydrodynamics of Arn-2 well and Arn-1 well is stronger than Fkh-1 well. There were various types of lithology in Qom Formation within these three wells, which were typically mixture of siliciclastic-pyroclastics-carbonate sediments. Through Wells facies,seismic facies and characteristic of lithology distribution integration analysis, the paper has pointed out that there exist seven sea-level cycle and five third order sequences in Kashan area (S1～S5). Lithology changes of Qom formation were controlled by eustasy change.4. There are two sets of active source rocks in Kashan area. One is Shemshak shale in Jurassic, the other is marl in Oligocene-Miocene Qom Formation. Source rocks in Jurassic have characteristics of secondary hydrocarbon generation. Firstly Source rocks enter hydrocarbon generating threshold at early Cretaceous, but the denudation stopped hydrocarbon generating in late Cretaceous. The basin subsidence in early Cenozoic made source rocks of Jurassic continue to generate hydrocarbon, and enter highly matured stage. Source rocks in Qom formation entered hydrocarbon generation stage for the basin quickly subsidence in late Cenozoic (about 4-5Ma), meanwhile source rocks in Jurassic have entered highly matured stage, generated lots of gas. The crude oil in well Arn-1 is mostly derived from Qom formation and other source rocks as well, belong to mixed oil reservoir. Hydrocarbon-generation amount of Qom Formation was not enough.5. The predominant reservoir of Qom Formation in Kashan area is marine carbonate rock, most of whom is bioclastic limestone. The reservoir is the fracture-pore reservoir with low porosity and low permeability. The rocks show brittle failure, and stress of well Fkh-1 in E member from AE testing was far less than compressive strengths of rocks . There is a serious heterogeneity of reservoir. Overall, C member of the reservoir was most developed. Among three wells, reservoir of the Arn-1 Well was most developed, followed by Arn-2 wells. Fkh-1 Well was the worst. According to the outcrop survey, the development of fracture was controlled by dual role of the fold and fault. The development of joints or cleavage related to lithology and thickness, the greater thickness, capable of better, sparser jointed, on the contrary, joint or cleavage more developed. Aran anticline in the plane C4 sub-mermber can be divided into three compound fracture system. Aran anticline northwest dumping fractured reservoir development is the most favorable position.6. The formation time of Aran traps is synchronization or late of oil and gas generation, migration, it is can effectively capture of oil and gas. Based on accumulation condition analysis of the Aran Structural, hydrocarbon and traps period of the good match is conducive to the oil and gas gathering. In adequate oil and gas sources and to accurately predict the distribution of fracture, compared to other area, Aran anticline is the most favorable target for exploration...