Research On Tectonic Paleogeomorphology And Sedimentary Facies Of Middle-late Jurassic In Bonaparte Basin, Australia

As a typical passive continental margin and multi-cycle superposed basin, bonaparte is a basin of vast area about 27×104km2 and its exploration area is about 27×104km2. All through the petroleum exploration of over 20 years, there are about 70 discoveries of oil and gas until July 2007, including Katandra oil field discovered in 2004 and gas field of Vesta,Caldita and Firebird discovered in 2005. Explored gas reserves of six big gas fields is totally of about 28800 Bscf while until now only eleven oil and gas fields have been or being developed. So Bonaparte basin is rich in hydrocarbon resources, low degree of prospecting and high potential for exploration. The post-fault thermal subsidence after Valanginian stage in late Mesozoic affected the palaeogeomorphology in tectonic units during synrift phases. So at present there isn't a uniform regional stratigraphic classification model which affected the stratigraphic correlation, paleogeomorphology restoration and sedimentary facies analysis. During middle-late Jurassic, bonaparte basin was under intense crack trapped period and undergone large-scale differences in rising, denuding and subsiding at callovian breakup unconformity. What mentioned above cause the differences in structure and topography among those tectonic units and affect source gallery, distribution patterns and therefore impact distribution patterns of sedimentary facies. So it is of great importance on prediction of favorable reservoir facies belts to proceed in establishing of chronostratigraphic sequence framework and analyzing of palaeogeomorphology, depositional system and distribution patterns of sedimentary facies between different tectonic units.The dynamic regime and structural evolution characteristics showed that the effects on tectonic patterns and sedimentary caused by tectonic activity is quite various during different evolutionary phase. Mid-late Jurassic is the main stage of basin synrift phases. During this period bonaparte basin is located in a relative stable neritic shelf environment with some locally abysmal area. Bonaparte basin is charactered by obvious inheritance deposition and favorable conditions of lithologicalpool.This paper applied principles and methods of sequence stratigraphy in marine facies about passive continental margin. Guided by structural geology, sequence stratigraphy, sedimentary, basin analysis and petroleum geology, based on structural evolution, litho-stratigraphic development and sedimentary filling, based upon seismic and well-correlation profile, a large-scale framework of sequences was established. And then to divide the sequence into a lesser level in Sahul Syncline. From aspects of typical seismic facies, electrofacies, single well facies and core facies, this paper deeply analyzed the tpical depositional characters of our purpose intervals and then concluded the distribution characteristics of every sequence sedimentary facies between different tectonic units. On that basis, combined with some rock ore and paleontology informations, we realized that there are extensive sources and various supplying directions of one source provenance. For example, Londonderry Uplift has the effect of source supply for Vulcan graben, Sahul Syncline, Malita graben, Petrel depression, which is rich in quartz. And Sahul Uplift supplies sources for graben mainly, Ashmore Uplift mainly supplies sources for Vulcan graben as well.Based on source analysis and cognition of struction and sedimentary background, this paper makes ancient appearence recovery for Jurassic and early Cretaceous formation, which results in the tectonic characteristics of the area with alternating uplifts and depressions. In the SQ1-SQ2 depositional period of early-middle Jurassic, depositional centre is in the north-central basin, which is located in Vulcan graben, Sahul Syncline, distal Petrel depression and Malita graben, and there are great differences on formation thickness. In the of middle-late Jurassic, Sahul Uplift became to arose and the area of sedimentation units expand greatly. This period the basin is characterized by inherited depositon and obvious weakened uniform settlements. When it comes to SQ3-SQ4 depositional period of early Cretaceous, Bonaparte basin also demonstrated as inherited depositon and along with large-scale of transgression which brought about expansion of depositonal areas.After comprehensive analysis above this paper proposed five kinds of sedimentary filling characteristics which match to dynamic mechanisms and summarized three types of typical sedimentary model which were delta, gravity current and bar phase respectively. And the discovery of bar sand body had an important guiding significance for exploration deployment of lithologicalpool. Then we concluded the distribution characters of sedimentary facies in midlate Jurassic based upon source, sand content, regional seismic facies and paleogeomorphology restoration. At the same time, we've also got the source and causes of sand body according to ancient flow and sedimentary microfacies of some typical single wells.In addition, brief analysis of diagenesis in mid-late Jurassic displayed its multi-stages and complication. Besides, we chose the favorable reservoir facies belts, such as submarine fan distributed in Vulcan graben, mouth bar and distributary channel of delta front developed in Sahul Syncline, Petrel depression and Malita graben and bar sand body located in the margin of the lifts, front and side of deltas. Bonaparte basin possessed good hydrocarbon-generation potential as its inherited depositon.