Characteristics And Genesis Of Lacustrine Sandbody Controlled By Paleogene Sequence Stratigraphy Framework In Tangyuan Fault Depression, Heilongjiang

Most reservoirs in continental sedimentary basins are all kinds of sandbody sedimentation in lakes, at present the sandbody which sedimentary genesis are relevant to lakes are called lacustrine sandbody. The distribution range of lacustrine sandbody is broad and the types are various, the deposition is complexly controlled by lacustrine facies belt. Synthetically Utilizing the theories and methods of sedimentology, sequence stratigraphy, geophysics and geochemistry, here we concentrate on studying the genetic types and sedimentary characteristics of Paleogene lacustrine sandbody in Tangyuan fault depression, then discuss their distribution regulars and genetic mechanism in isochronic sequence stratigraphic framework. This research could service for petroleum geologic work in research target and enhance the forecast and success rate of oil and gas exploration.Yishu Graben is one branch of Tan-Lu fault to north, which trends NE and crosses throughout Northeast China. Tangyuan fault depression is the north sub-tectonic unit of Yishu Graben, which is a dustpan-like fault depression, faulting in the east and overlapping in the west, and is controlled by eastern border fault F1. Inside Tangyuan fault depression, the sedimentary fills are mainly Paleogene formations, faults are very developed. The fault depression has structural framework with east-west zoning and south-north blocking, which could be divided into five sub- structural units from east to west that is eastern strike-slip thrust zone, eastern hollow zone, center bossy zone, western hollow zone and ramp region, and five relatively independent blocks with obvious differences from north to south that could be further divided into fifteen second-order structural units and several third-order structural units. The basement characteristic, modality and fault activity of every block is different, which causes every secondary depression has the features of different geologic characteristics and evolutional history.Here we adopt the division concept of nine-class sequence unit (Van Wagoner et al, 1990) to do research on Paleogene sequence stratigraphig of the Tangyuan fault depression. Aim at third order sequence, we adopt the technical terms of four-division system tract within a third order sequence(Liu Zhaojun et al, 1994, 1997, 2002).Using core, outcrop and geophysics as main research methods, palaeontology, geochemistry and mineralogy as supplementary research methods, by synthetically analyzing a lot of materials of drilling cores, filed outcrop, logging, seismic data, palaeontologic fossils and test data of clay minerals, heavy minerals and geochemistry, we identifies the Paleogene sequence stratigraphic units of this area and marks twelve Sequence boundaries that is T₁, T₁¹, T₂, T₂¹, T₂², T₃, T₃¹, T₄, T₄⁰, T₄¹, T₄², and T₅, and summarizes the characteristics of sequence boundaries from megasequence to parasequence and their identified marks. Among them, T5 is the megasequence boundary, T₁ is supersequence sets boundary, T2, T3, and T4 are supersequence boundaries, T₁¹, T₂¹, T₂², T₃¹, T₄⁰, T₄¹, and T₄² are third order sequence boundaries.Based on distinguishing and dividing every sequence stratigraphic units, here we carefully analyze and study the system tracts, parasequence sets, and parasequence characteristics within third order sequence. According to observation and description of drilling cores and outcrops, this research interprets fifteen kinds of lithologic and sedimentary structural types, which makes up different sedimentary microfacies to explain the vertical inner characteristics of parasequences. According to the superimposed patterns of parasequence, it could be divided into progradational,retrogrational and aggradational parasequence sets, and the grain size,silt ratio,well logs and signal parasequence depth of different types have apparently different characteristics in vertical changes.By studying the interface characteristics and identified marks of every order sequence stratigraphic unit of Paleogene in Tangyuan fault depression, Uniting with the internal structural characteristics of sequences, this research sets up the isochronal sequence stratigraphic framework of Paleogene in Tangyuan fault depression in seismic data, signal well and the connected wells profile, which is make up of one supersequence set, four supersequences, and eleven third order sequences. Among them, during fault period, supersequence set consistes of intense fault supersequence(SS1), persistent fault supersequence(SS2), supersequence transformed from fault to depression(SS3), and fault shrinking supersequence (SS4). Intense fault supersequence (SS1)contains four third order sequences that are SW1, SW2, SX1 and SX2, equal to Wuyun Formation and Xinancun Formation, persistent fault supersequence (SS2) contains two third order sequences that is SD1 and SD2, equal to Dalianhe Formation. supersequence transformed from fault to depression(SS3)contains three third order sequences that is SB1, SB2, and SB3, equal to the 1st member of Baoquanling Formation. fault shrinking supersequence (SS4) contains two third order sequences that is SB4 and SB5, equal to the 2nd member of Baoquanling Formation.According to drilling cores and field outcrops, united with logging, seismic, palaeontologic and geochemical data, and three kinds of sedimentary systems develope in this area that is fan delta, lacustrine, and underwater gravity current sedimentary systems, and twelve kinds of sedimentary subfacies and nineteen kinds of sedimentary microfacies can be further identified. From sedimentary characteristics of lithologic association, rock texture, primary sedimentary structure, geochemical response and sandbody morphology, we synthetically study all kinds of lacustrine sandbody developed in Paleogene of Tangyan fault depression and divide it into three genetic types of fan-delta, lacustrine and underwater gravity current. Among them, this is the first time to do research on sandbody of lacustrine genesis in this area. Moreover, learning from the past research results, here we firstly establish the systematical classification concept of subaqueous gravity current sedimentation of Paleogene in Tangyuan fault depression. According to formation mechanism and provenance supply factors, subaqueous gravity current could be divided into steady types and surging types. Among them, steady subaqueous gravity current sedimentation shows subaqueous fan with fan model, which could be further divided into abrupt subaqueous fan and gentle subaqueous fan by its tectonic location of distribution. Synthetically considering the dimensional distribution modality of surging subaqueous gravity current sedimentation, it could be divided into turbidite fan with fan model and turbidite with non-fan model, their formations are all related to the active intensity and style of faults developed in the region, and the formation of turbidite fan is effected by the degree of provenance supply controlled by the tectonic activity of syngenetic faults. Compared with steady subaqueous fan sedimentation, the scale of surging subaqueous gravity current sedimentation is smaller, so we could divide the internal microfacies and ascertain the plane distribution range according to the develop degree and completeness of Bouma series.Based on synthetically studying all kinds of sedimentary systems in isochronal sequence stratigraphic framework of Paleogene in this area, we recover the plane distribution characteristics of sedimentary systems in different structural stages, discuss and conclude the distribution regulars and formation mechanism of different types of lacustrine sandbody in sequence stratigraphic framework. In the period of Wuyun Formation and the 1st member of Baoquanling Formation, fan-delta sedimentary system developed in the two sides of the basin, subaqueous fan sedimentary system developed in the side of boundary fault controlling the basin, and coastal shallow lake sedimentation developed in the internal of basin. In the period of Dalianhe Formation and the 2nd member of Baoquanling Formation, fan-delta sedimentary system developed in the two sides of the basin, and coastal shallow lake sedimentation dominates over the internal of basin, semi-deep lake sedimentation developed only in the center of the west. Inside every third order sequence, LST is formed in the stage of forced rapid base-level fall, and base-level lies in the lowest. In the period of LST, the zones of steep slope and gentle slope with fault and slope-break develop mainly fan-delta sedimentary system, the types of sandbody in fan-delta plain and front zones are abundant, the eastern and western hollow zones develop partly subaqueous fan sandbody, the gentle slope with slope-break is exposed and eroded, and develops incised vally. TST is formed in the stage of rapid base-level rise. In the period of TST, the steep slope develops retrogradational fan-delta sandbody, the gentle slope develops retrogradational fan-delta sandbody and beach-bar sandbody, the periphery of the former center bossy zone develops beach-bar sandbody, the hollow zones of two sides develop subaqueous fan sandbody and surging turbidite sandbody in deep-water area. In the period of HST, base-level lies in the highest and be steady, the scale of fan-delta sedimentary system decreases and the fan-delta plain zone narrows down, the gentle slope develops beach-bar sandbody, the hollow zone develops steady argillaceous sediment of semideep and deep lake occasionally with surging turbidite sandbody. In the period of RST, base-level falls slowly, the two margin of basin develop progradational fan-delta sedimentary system dominated by fan-delta front sandbody, the former center bossy zone develops partly beach-bar sandbody, and the hollow zone develops sub-aqueous fan and surging turbidite sandbody. In continental fault depression basin, the factors controlled the sequence development is complex and diversified. By analyzing the buried history of Tangyuan fault depression in paleogene, we firstly use the Back-strip to quantitatively calculate the tectonic subsidence quantity and rates of every structural stage, semiquantitatively and quantitatively analyze the factors of controlling the develop of each order sequence, propose base-level change controlled by tectonic action, palaeoclimate, and sedimentary provenance supply forms third order sequence with different styles, and conclude every kinds of sequence styles developed in break of slope region, ramp region, palaeocentral uplift belt and depression belt.Based on analyzing the control factors of sequence development in depth, comparatively studying the typical examples of sequence stratigraphic models of other continental fault depression basins, taking third order sequence as basic sequence stratigraphic unit, we firstly propose the sequence stratigrphic model of compound half graben with palaeocentral uplift. Uniting the development characteristics of fault in different structural stage of Tangyuan fault depression and the interaction of the supply of sediments and accommodated space, we subdivide the sequence stratigraphic model into supercompensated, compensated and undercompensated, which separately display the types and characteristics of sequence developing in different structural stage of fault depression basin. Among them, supercompensated model is on behalf of sedimentary sequence with fan-delta and coastal shallow lake sedimentation developed in the early stage of intense fault, when tectonism actived intensely, the rate of subsidence is big, but the supply rate of sediments is bigger than the increase rate of accommodated space. In the sequence, LST and TST are most developed, the second is RST, and the develop scale of HST is smallest. The bottom and top of sequence are mostly scour surface with intense downcutting. In the margin of basin, every system tract develops mostly progradational fan-delta sedimentary system displayed by coarse sediments of several stages of distributary channel which superimpose and cut each other. In the internal of basin, LST develops shallow lake sedimentation, but the other systems tracts develop mainly semideep lake sedimentation. Totally it is shown that the water depth of basin becomes small, and the lake sedimentation depauperates. Compensated model is on behalf of sedimentary sequence with fan-delta and semideep lake sedimentation developed in permanent fault period, when tectonism actived intensely, but the rate of subsidence is small, the supply rate of sediments is equal to the increase rate of accommodated space. In the sequence, TST and RST are developed, the develop scale of HST is smaller, and LST is absent in some areas. The bottom and top of sequence are mostly scour surface and erosional surface. In the margin of basin, every system tract develops fan-delta sedimentary system with apparent cyclicity. In the internal of basin, coastal shallow lake, semideep lake and subaqueous gravity current sedimentation all develop, however, subaqueous gravity current sedimentation mainly develops in the period of TST and RST. Totally it is shown that the water depth of basin has no big change, and the lake sedimentation is steady. Undercompensated model is on behalf of sedimentary sequence with fan-delta and deep lake sedimentation developed in the period transformed from fault to depression, when tectonism actived stably, the scope of subsidence is wide, and the supply rate of sediments is smaller than the increase rate of accommodated space. In the sequence, HST is most developed, the second is TST and RST, and LST is undeveloped. The bottom and top of sequence are universally conformable surface in the internal of basin, and are unconformable surface in local area. In the margin of basin, every system tract develops retrogradational fan-delta sedimentary system displayed mostly by fan-delta front sedimentation. In the internal of basin, dark colored mudstone of deep lake is most developed, but in the period of LST,TST and RST, subaqueous fan and surging turbidity current sedimentation are also developed. Totally it is shown that the water depth of basin and the scale of deep lake sedimentation becomes big. Base on above research, we discuss the formation mechanism of every kinds of lacustrine sandbody in sequence stratigraphic framework with different model.