The Sedimentation And Hydrocarbon-accumulation Effects Of Paleogene Tectonic Evolution In Qingdong Sag

Qingdong Sag is a Paleogene terrestrial fault basin next to Tanlu fault zone. With abundant resource potential, it is one of the significant succeed bases of reservoir increasing and production stabilization of Shengli oilfield. In the early stage, the exploration on structural traps was primarily carried out in the study area. There were very few studies on the development and characteristics of sedimentary system, which limited the progress of exploration on oil and gas reservoirs. Tectonic background controlled the development of sedimentary system and the accumulation process of oil-gas, thus the study on the sedimentation and hydrocarbon-accumulation effects of tectonic evolution is significant both in theory and practice for determining the law of petroleum accumulation and promoting the exploration of reservoirs especially the subtle reservoirs.In this thesis, multidisciplinary theories including structural geology, seismic sedimentology, sequence stratigraphy, sedimentary petrology and petroleum geology etc. were utilized. Based on the previous research results and abundant essential first-hand data, such as 2D/3D seismic data, mud logging data, well logging data,well testing data, and laboratory data etc. the tectonic settings and tectonic evolution process of the study area were analyzed and the Paleogene sequence stratigraphic framework was established firstly. Then, the main sedimentary facies was identified,the development law of the sedimentary system of mainly oil-bearing formations was analyzed. Based on that, the sedimentary response to the tectonic activities was analyzed so as to establish the tectonic-controlling sandstone development models. On the basis of the studies above, the effect on hydrocarbon accumulation of the tectonic and sedimentary evolution was further studied to determine the law of hydrocarbon accumulation and its main controlling factors. Finally, the hydrocarbon accumulation patterns were established and the favorable exploration direction of subtle reservoirs was pointed out.Based on the studies above, significant progress and innovative knowledge were obtained as follows.(1) The formatin process of the Palaeogene fault basin of Qingdong Sag can bedivided into three episodes, including initial fault-depression episode, intense fault-depression episode and dormant fault-depression episode. Based on the characteristics of sequence stratigraphy in different episodes of fault-depression, the Palaeogene formation was divided into three second-order sequences including six three-order sequences. There are several reservoir sedimentary types in Paleogene formation, such as alluvial fan, neritic subaqueous fan, fan delta, delta, beach bar and turbidite fan, etc.(2) At the initial fault-depression episode, the lower Es4 Member was developed,the sedimentary system in which was mainly composed of alluvial fan from the west and north provenance and diffuse lake beach bar. At the intense fault-depression episode, several types of depositional systems, such as the neritic subaqueous fan, fan delta, delta, beach bar etc. were developed along the periphery of the depression in the upper Es4 Member, delta from eastern and southern provenance and fan delta from western in the middle and lower Es3 Members, and braided river delta from eastern and southern provenance in the upper Es3 Member and Es2 Member. At the dormant fault-depression episode, and Es1 Member and Dongying Formation were developed,the main depositional system in which is braided river delta from eastern and southern provenance.(3) Tectonics controls on the sequence pattern and the depositional law was mainly manifested as follows. Firstly, stratigraphic distribution of the third-order sequence was controlled by the tectonic framework of different fault-depression episodes. That was NW trend faults at the initial fault-depression episode,approximate EW and NNE trend faults at the intense fault- depression episode, and NNW trend faults at the dormant fault-depression episode. Secondly, Paleogene subsidence and sedimentary centers migrated from SW to NE controlled by regional tectonic NS tilting. That led to the early main source direction from north to south turning into south to north later. Thirdly, fault ditches developing at denudation zone and boundary fault accommodation belt controlled the main passage of terrigenous detrital material from provenance to lake. And the internal growth fault controlled the distribution of terrigenous clastic sedimentary system. Finally, the underwater highland with various origins influenced the reallocation of terrigenous detrital materials.According to the control of tectonics on the development of sedimentary system,five sandstone devolopment patterns were established, which are the boundary fault section concave channel-fan pattern at steep slope, the paleotopography channel-fanpattern at gentle slope, the fault strike channel-fan pattern on the hanging wall of internal growth faults, the slope-fan pattern at strike-slip fault accommodation zone,and the underwater highland- beach bar pattern at the uplift belt in the sag.(4) The whole process of oil-gas accumulation was controlled by the tectonic and sedimentary evolutions in Qingdong Sag. Firstly, Source rocks were mainly developed in the upper Es4 Member and the middle and lower Es3 Member at the intense fault-depression episode. The thickness of source rocks, the organic type and abundance were controlled by the tectonic framework of the sag. The north-central sub-sag was the subsidence and sedimentary center as well as the hydrocarbon generation and expulsion center. Secondly, at the main hydrocarbon accumulation period, which is from Neogene Pliocene to Quaternary, oil-gas migration in vertical direction was obstructed due to the weak activities of faults, while the lateral contact of source rocks with a variety of sand body was advantageous to the lateral migration of oil-gas. The tectonic and sedimentary characteristics determined that hydrocarbon migrated primarily in horizontal direction in Qingdong Sag. It followed that the main oil-bearing stratas are consistent with the main source rocks. Thirdly, the hydrocarbon distribution was confined by both the priority pathway of hydrocarbon migration controlled by the contemporaneous faults and the skeleton sand bodies related to them and the reservoir and seal assemblages in the different tectonic belt controlled by the vertical and horizontal changes of the sedimentary systems. Finally, hydrocarbon accumulation and enrichment was controlled trap conditions directly. Because most faults in Qingdong Sag are approximate EW strike, the size of single structural trap and the total scale structural traps such as fault noses and fault blocks are relatively small. And the lateral sealing conditon of the stratigraphic traps in the margin is unfavorable because of the angular unconformity between the top of Paleogene which experienced long denudation and the bottom of Neogene formations which consist of sandstone in block. Lithologic trap within the source rock has great exploration potential in the sub-sag belt.(5) According to the differences in petroleum geology conditions, reservoir characteristics, the migration and accumulation styles, the law of hydrocarbon enrichment and the master factors of reservoir-forming controlled by the tectonic and sedimentary evolution, four hydrocarbon migration and accumulation models were established, including multi-dimensional migration and accumulation model at steep faulted belt, source-adjacent convergence accumulation model at uplift belt in the sag,fault-sandstone-strike migration and accumulation model at slope belt, and in situgeneration and accumulation model within the sub-sag. The in situ subtle lithologic reservoir within the sub-sag is the main exploration target in the future.