| The Bonaparte Basin, located in the mainland and the continental shelf of northwestern Australia and developing from Cambrian to the present is a large fan-shaped rift basin whose area is more than270,000km2, including both onshore and offshore parts. The Bonaparte Basin is a typical oil-rich basin of the North West Shelf of Australia and has sufficient hydrocarbon sources and complex hydrocarbon accumulation conditions, of which the Jurassic source rocks, characterized with organic matter abundance is the main source rocks in the basin and also the object of this study. Geochemical characteristics of source rocks are evaluated according to organic matter abundance, type and maturity. By integrated use of geological, geophysical and geochemical data and basin modeling technology, the buried history and thermal history in research area are recovered to be combined with EASY%R。provided by software to complete the one-dimensional and two-dimensional maturity history recovery of the Lower Jurassic and Middle Jurassic source rocks. On the basis of geochemical characteristics and the maturity history, hydrocarbon generation and expulsion history are caculated.This Paper consists of seven chapters:The first chapter is the Introduction; The second chapter describes the regional geology of the study area; The third chapter describes the geochemical characteristics of Jurassic source rocks; The fourth chapter discusses the one-dimension geological history simulation of some typical single wells in Bonaparte Basin; the fifth chapter describes the simulation methods and parameters selection of the thermal evolution history;Chapter6discusses hydrocarbon generation and expulsion history in different tectonic units as well as hydrocarbon generation and expulsion contribution in different layers of source rocks.According to the above research, the paper has made the following understanding:By statistical analysis of the source rock geochemical parameters, the results show that:1) the source rocks in Plover Formation of LowerJurassic belong to the good level, with high organic matter abundance and and an average organic carbon content of3.54%. Source rocks quality of the Elang Formation and Frigate Formation of Upper and Middle Jurassic with an average organic carbon content of1.44%and1.38%, respectively, is slightly worse than that of the Plover Formation in the Lower. Simutaneously, chloroform bitumen "A", total hydrocarbon and statistical analysis of hydrocarbon generation potential also show the same characteristics in Hetaoyuan Formation sourcerock; By kerogen elements and rock thermal solution, the types of organic matter are analysed, of which the Jurassic source rocks in the Petrel Sub-Basin, Ashmore Platform and Malita Graben are mainly type Ⅲ and type Ⅱ2, while are Ⅱ2-oriented in Vulcan Sub-Basin, Sahul Syncline, Londonderry Highland and Sahul Platform, including a certain number of type Ⅲ, type Ⅱ1and a small amount of type Ⅰ. By vitrinite reflectance and pyrolysis peak temperature data, the maturity stages of Jurassic source rocks are partitioned, most of which are in the low mature-mature stage and generally enter low mature, mature and high mature stages in depth of1500m,3000m and3500m separately. On the whole, negative tectonic units of the Vulcan Sub-Basin, Sahul Syncline and Malita Graben has relatively higher matrurity, especially the Malita Graben, most of which is at high mature and even overripe stage. ripe, even overripe stage, whereas the matrurity of positive structural units of Londonderry Highland, Sahul Platform, Ashmore Platform, and Paleozoic sedimentary-based Petrel and Paleozoic sedimentary maturity is relatively lower, basically at the immature-low mature stage.The simulation of geological history recovery shows that:different regions in Bonaparte Basin have different sedimentary and deposition characteristics in different tectonic evolution period. Lower Jurassic (Plover group) was the slow rift and depression stage with lower basement subsidence curve; At the end of sedimentary in Middle and Upper Jurassic (the Elang and Frigate), Bonaparte Basin underwent a fast fault depression, resulting in the Middle and Upper Jurassic strata basement subsidence curve being steep and the slope increasing substantially, which reflects a widespread rapid subsidence in Middle and Upper Jurassic strata. For the dynamic mechanism revolved from extention to compression, the tectonic framework of Bonaparte Basin has experienced uplift and denudation since the end of late Cretaceous which varied according to regions (see above) and the basin subsided to redeposite at about90Ma.The present geothermal gradient of Bonaparte Basin is higher with an average one of about3.2℃/100m and the geothermal gradient in gentle slope belt is higher than that in deep depression and abrupt slope. Heat history based on the heat value nowdays whose average is55.4mW/m2can be divided into two phases:the basin was at fault-depression experiod from early Jurassic to early Cretaceous during which mantle rose through basement to conduct heat with layers that made the heat value keep increasing and reached a maximum at about160Ma, after which the basin subsided steadily until late Cretaceous when it entered structure reverse period with uplift before another steady subsidence and redeposit. Up to now, the heat value declined gradually.The maturity history simulation of typical single wells show that:the maturity history varies greatly with different tectonic units and different source rocks. In the northern deep depression, taking well Heron1as an example, the Lower Jurassic (Plover group) source rocks entered low maturity, medium maturity and reached hydrocarbon generation peak in158Ma,88Ma and34Ma, respectively. Middle and Upper Jurassic (the Elang group and Frigate group) correspodingly entered low maturity and medium maturity in96Ma and81Ma, and now in the medium mature-high mature stage. In the southern deep depression, taking well Petrel2as an example, lower Jurassic source rocks began to enter the hydrocarbon generation threshold in about86Ma, and just reached the medium maturity now; Middle and Upper Jurassic source rocks just entered low maturity in about31Ma and is in the immature-lowmature stage today.Two-dimensional Maturity history simulation indicates that:the Lower Jurassic (Plover group) source rock has the highest maturity and, except for the southern half of Londonderry Highland and Petrel Sub-Basin, are generally in mature-highmature stage which beongs to high matrue in Vulcan Sub-Basin and deep depression in south Sahul syncline and high mature-overmature in Malita Graben. On the whole, east and north region has higher maturity than west and south region. Middile and Upper Jurassic (the Elang group and Frigate group) source rocks is at mature stage in south Sahul Syncline, Malita Graben, the northern half of Petrel Sub-Basin and the western half of Vulcan Sub-Basin with most parts of Malita Graben being at high mature stage and overmature area in the western half of Vulcan Sub-Basin being bigger. Overall, except that the southern half of Londonderry Highland and Petrel Sub-Basin is still at immature stage, rock sources in other tectonic units have already been mature and generally, maturity of the east and the north is higher than that of the west and south.Hydrocarbon generation and expulsion intensity history shows that:in geological history, the Lower Jurassic source rocks generate and expelled much more hydrocarbon than the Upper Jurassic source rocks and the extrusion uplift tectonic event that happened at the end of Fanlanyin Sedimentation has important impact on the hydrocarbon generation and expulsion. |
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