Abstract
In this study, One and two-dimensional models of petroleum systems were constructed in the internal part of the Fars Region. Based on the results obtained from One-dimensional modeling and the study of thermal maturation history, the Sarchahan Formation is considered the main source rock of the Paleozoic petroleum system, which started hydrocarbon generation about 130 to 140 million years ago. The liquid hydrocarbon content in wells A and B is estimated to be 720 and 5100 (Kg/m2) respectively, and the gas hydrocarbon content is estimated to be 70 and 600 (Kg/m2). Furthermore, based on the results of two-dimensional modeling, the younger petroleum system source rocks such as the Cretaceous and younger source rocks are thermally immature and have not entered into the oil window. According to the migration modeling results, the most important factor in the movement of hydrocarbon fluids prior to the Zagros orogeny is vertical migration in the career layers, but after this orogeny and tectonic compression, the horizontal migration compOnent has also become active.
Keywords: Petroleum system Modeling, Burial history, Source rock, Fars internal Region
Introduction
The petroleum system modeling functions as a powerful tool in the process of hydrocarbon exploration, where Regional studies and simulation of subsurface processes can significantly assist in identifying traps and exploration targets. A petroleum system consists of various elements including source, reservoir and cap rocks, hydrocarbon traps, and processes such as generation, migration, and hydrocarbon accumulation. Modeling of petroleum systems is divided into One-, two-, and three-dimensional (1–3D) based on the specific objectives and available data. One-dimensional modeling, nowadays, is used as a computational method based on measured data to indirectly evaluate the maturity of source rocks and determine the oil and gas generation window. For a better understanding of the petroleum system, especially during hydrocarbon generation in the source rock and subsequent expulsion of hydrocarbon molecules, One-dimensional modeling has special applications. The purpose of this study is to construct 1D and 2D models of the petroleum system of two fields located in the Internal Fars with the use of data from two exploration wells and a 2D structural section in order to determine the liquid and gas hydrocarbon content generated from the source rock, as well as the migration pathways of various hydrocarbons over geological timescales.
Material & Methods
The first step in One-dimensional modeling is to create a well and input relevant data such as well name, geographical coordinates (longitude and latitude), final depth, and rotary table height in the dedicated section. The next step is to input data related to different layers, including information such as layer name, depth, thickness, and age of the layers, palaeo water depth, erosion rate or non-deposition time, lithology related to each formation, the role of each layer in the Regional petroleum system, temperature data, and finally, geochemical data such as total organic carbon (TOC) and kinetic data related to potential source rocks. =The TOC and hydrogen index values should be considered initially to ensure calculations are performed with minimal error. In order to simulate a sedimentary basin, the following parameters need to be defined for each event or geological layer. After constructing and running the One-dimensional model, calculations related to temperature and maturity are calibrated using measured data, and then the modeling results are extracted. These results include the burial history plot of strata at the well location and time plots of various parameters of source rock such as vitrinite reflectance, hydrocarbon generation and expulsion rates for potential source rocks in the study area. The first step in the two-dimensional modeling of the hydrocarbon system is to input the structural pattern (2D structural interpretation from seismic sections) into the software. To do this, depth lines representing layers with structural interpretations in the cross-sectional direction are first inputted into the software, and other layers are defined using the lithological pattern and isopach maps. In the software Open-Flow, it is necessary to reconstruct the layers up to the basement, so all successions and studied formations should be included in the model, from the surface and topographic layer to the basement. In this study, information on sedimentology, lithology, thickness and erosion rates of layers, palaeo-water depth, and organic geochemistry of two wells located in two gas fields was utilized. On the other hand, for determining the structural pattern of the study area, a 2D structural cross-section interpretation prepared perpendicular to the Zagros thrust trend was used. By constructing a 1D model in two selected wells, the maturity and hydrocarbon generation history of the source rocks were evaluated. Additionally, the heat flow of each well was calibrated with measured temperature data, and based on these data, a heat flow map was designed in the 2D models.
Discussion of Results & Conclusions
To achieve more accurate results in modeling, hypothetical layers from the bottom of each well to the top of the basement were considered. On the other hand, a layer reconstruction process was carried out for a portion of the Aghajari Formation which had partially eroded. Based on the data and various tested models, the thermal flow of the basin with a good match to the current well temperature was assumed to be a constant of 55 mw/m2. Considering the very good match between the measured temperature in the well and the calculated temperature in the model, the modeling results can be reliable. The obtained results showed that the Sarchahan Formation began hydrocarbon generation approximately 130 to 140 million years ago, with liquid hydrocarbon amounts of 720 and 5100 (Kg/m2) in wells A and B, respectively, and gas hydrocarbon amounts of 70 and 600 (Kg/m2) respectively. By constructing a 2D model in the selected structural section (perpendicular to the Zagros fold-thrust trend), migration and accumulation paths of hydrocarbons were identified. The results obtained from the 2D model indicate that the Paleozoic oil system was formed with the Sarchahan Formation as the source rock, the Dalan and Kangan formations as reservoir rocks, and the Dashtak Formation as the cap rock. The Sarchahan Formation is the most important source rock that charges the Regional reservoirs, and the Cretaceous source rocks had a minimal role in hydrocarbon accumulation due to low maturity. The thermal maturity of the Sarchahan Formation is currently in the late oil window at the top parts of the anticlines and in the wet and dry gas window in the base of the synclines. According to the migration model results, hydrocarbon migration to the reservoirs has started from the Late Cretaceous. Prior to the Zagros orogeny and considering the results of structural reconstruction, there was no folding in the sediments, and throughout the section, the layers were stacked on top of each other like a Layer Cake. Therefore, before the orogeny, the most important factor in the movement of hydrocarbon fluid migration was vertical migration in the career layers. After the orogeny and folding, in addition to vertical migration, the horizontal migration compOnent became active.