| The Bonan Sub-Sag is located in the north-central part of the Jiyang Depression, and is rich in oil and gas. The sha-3 and upper sha-4 members of Shahejie Formation are the main oil-bearing series which has 210 million tons of proven reserves. The Bonan Sub-sag has been explored very well in general, but the lower sha-4 member of Shahejie Formation never got enough attention, which makes it always as the low degree exploring layer system. As the oil and gas exploration gets deeper and more thorough, finding for new reserves has become more and more difficult. However, the Luo-68 well which finished in the spring of 2010 has drilled 10.8 m /4 oil layers in the red beds of lower sha-4 member of Shahejie Formation; it outputs 71.4 tons of oil per day in the period of well testing, which shows great exploring prospects. In this paper, By combining basic data such as seismic, drilling and experiment, as well as systematically studying the structural evolution of red layer, subtly stratigraphic correlation, sedimentary feature of red layer, as well as distributing regulation of reservoir and petroleum reservoir characteristics of lower sha-4 member, this paper illustrates the reservoir characteristics, petroleum reservoir characteristics and accumulating regulation of red layer in the research area. On the basis of comprehensive evaluation, we find out the advantageous explore direction and potential place, research results also have acquired various exploration achievements, new exploration layer system has been found, which makes the red layer-bearing measure become the important reservoir bed of the Jiyang Depression and even Shengli exploration area.The research reaches following results and conclusions:1. The tectonic framework and tectonic evolution characteristics developed in the red layer sedimentary rocks are clarified, and the tectonic framework of red layer original basin of Bonan Sub-Sag is restored for the first time. Guided by the principle of sequence stratigraphy, through the combination of well-log and seismic data and analysis of structural evolution profile, the palaeogeomorphic tectonic framework of lower sha-4 member and other members are presented, laying a good foundation for the understanding of basin sedimentary reservoir.1) Multi-stage of tectonic evolution. Since the Mesozoic era, stress field change happened several times, the Bonan Sub-Sag stays in the center of Zhanhua Sag during in Paleogene period.2) The difference between fracture systems. The differences of fracture systems between lower sha-4 member and upper sha-4 member of Shahejie Formation are defined. The tectonic evolution process of lower sha-4 member is an important structure transition period, so the difference exists as the change of tectonic stress field in lower sha-4 member and upper sha-4 member of Shahejie Formation. The fracture system of lower sha-4 member is mainly in the south of Bohai South Sub-Sag, the fault downward cut-through the layers of Kongdian Formation and Mesozoic, controls the sedimentation from the Kongdian Formation to the lower sha-4 member of Shahejie Formation; but the fracture system of upper sha-4 member of Shahejie Formation is mainly in the north of Bohai South Sub-Sag in the south-central which has some controlling effects on reservoirs of Paleocene.3) The matching of palaeogeomorphology. Several palaeo-gullys developed in the periphery of palaeogeomorphology, which controlled the injection direction of provenance, lays the foundation for the understanding of the provenance system; the palaeo-gully match the ancient landform type, and present the macroscopic distribution rule of sand body.4) The migration of depocenters. The thickness center of layer controlled by the north Guxi border fault, and it is north-west trending along with this fault; the center migrated from the northeast to the northwest come from below.2. Various methods are applied to establish the method of stratigraphic division and contrast which is suitable for barren strata in the Bonan red layer formation. By comprehensive utilization of lithology, logging, seismic data with cyclostratigraphy, lithostratigraphy and sequence stratigraphy, the stratigraphic contrast of the fourth member of Shahejie Formation has been built. The fourth member of shahejie formation of Bonan depression have divided into one sequence stratigraphy, and the sequence can be subdivided into three mid-cycles(fourth sequence). Base on this method, stratigraphic division of the 65 boreholds, with well-tied sections were used to construct the regional stratigraphic framework. On this base, later evolution and strata contact relationship of red beds could make sure in each sequence unit. It presented the characteristics of downlap on the bottom and truncation on the top strata in the Zhanhua sag.3. The sedimentary characteristics of red beds are illustrated, and deposition features of multi-facies existed in the same phase is clearly analyzed. In this paper, the sedimentary model of dry-salty lake in the Bonan area has been established. The results change the traditional knowledge that this formation was consisted of "red" strata and the sedimentary type was single without stable water system. Compared with previous points, some new views are as followed:1) Revised the mudstone formation color. Predecessors thought the climate is dry in the depositional period of the fourth member of Shahejie Formation, moreover the evaporation was big. There is no stable water, under oxidation environment for a long time, so the forming mudstones are in red. However, Under the guidance of new geological knowledge, recently, the boreholes of the yi-185,yi-186 and yi-193 drilling a large set of dark strata. For example, yi-185 well have drilled 350 meters in the fourth member of Shahejie Formation, but the colour of mudstone were still grey, dark gray or black. Above understanding have changed the past points that mudstone color were red. Above the water level, alluvial-fluvial facies were mainly developed with purple mudstone or red mudstone, gradually to the north, transferred to grey under the water level. All in all, the colour of mudstone presents gradually darken from the south to the north.2) Enrich the sedimentary type. Anterior cognition deemed alluvial fan and alluvial plain sedimentary system was the only type in the Bohai region. in contrast, in fact in this formation sedimentary types were various, except the development of alluvial fan-alluvial plain, there also developed shallow lake delta, beach/shoal sand and the nearshore subaqueous fan and fan delta from steep slope zone in northern.3) Discovered the inheritance of salt lake. Due to the Chengnan, Guxi border fault control, the northern boundary fault (down plate), developed stable water and had successive salt lake sedimentary, the lake range has the characteristics of gradual expansion from bottom side to top side.4. The main sedimentary factor of 4th Member of Shahejie Formation is revealed, and "five ancient sand control" of sedimentary origin mechanism is defined.1) Paleoclimate change controls the type of deposition.1). Different backgrounds of climate possess different climatic deposition type combination. There are significant differences between the higher sub-segment and lower 4th Member of Shahejie in the research areas, and thus created a different sediment types.2) Antiquities source control scale deposition body. During the deposition of 4th Member of Shahejie Formation, the peripheral edge of the salient are capable of providing material sources, including southern Chenjiazhuang source for the strongest, and the biggest fan body is formed by its offer, while the west Chuang Yi salient possess the minimum capacity for the source. The strength of the source material supply capacity determines the size and scope of the periphery of the fan and the capacity of body propulsion.3) The depth of the ancient water and the ancient salinity control the distribution of sedimentary body. Elemental qualitative analysis is used to analyze the depth of the ancient water and the ancient salinity Analysts believe that the depth of ancient water in the northern part of is relatively large. Paleosalinity is generally greater than 15‰, for brackish water depositional environment, provenance systems carried by various salts converge to the low-lying northern slope. Northern steep slope is the deepest and highest water salinity areas, salinity and the depth of water joint control the plane distribution rules for each type of sedimentary facies; 4) Palaeogeomorphic form controls the Macroscopic distribution law of the sand body. Multiple directions provenance system is developed. Its provenance is mainly consist of Chenjiazhuang, Chendong and islands salient. Yihezhuang salient is for the secondary source area; On the fault downthrown side, which is general close to the provenance of the whole basin margin, develop thick layer of sand body, sand thickness is thinning toward the center, and converge in the center of provenance. Various projections were developed multiple ancient gullies, with Various provenances are developed multiple ancient gullies, with"ditch versus fan", "big ditch versus big fan" and "small ditch versus small fan" deposition characteristics. Control by the western boundary of the northern part of the fault, the catchment area concentrated in the northern courtyard south-Guxi downthrown fault, showing Guxi faults is along with NWW trending faults Developed two-three thickness center, deposition centers, from the bottom up, have migrated from the northeast to the northwest.5. The petroleum geological conditions of red layer series are further clarified.1) For the first time, source rocks in Sha 4 lower submember have been discovered. Early researchers believe that the whole Sha 4 lower submember is red layer and has not developed a stable water body. This strata doesn’t have conditions for oil generation, and oil and gas come from the Sha4. Under the new guidance of new geological understanding, according to the analysis of the oil wells, we believe that it has the characteristics of self-generating and self-storage. Oil source come from the lower sub-section of Sha4. The new discovery of source rocks changes the traditional understanding of the Bonan sag even Shengli Oilfield has strategic significance. During the deposition of 2 sand units, greater thickness is mainly concentrated in the edge and central areas of the sag.1 sand units generally eroded, residual reservoir distribution in the plane, showing superimposed contiguous features. Reservoir thickness greater than 40m high has the characteristics of contiguous, located in north-central sag.6. The Sha 4 lower submember mainly develops structural reservoir as well as complexity reservoir formed by structure-lithology-stratigraphy, and the main controlling factors for oil and gas enrichment in the study area are revealed. The main control factors include the following aspects:1) oil and gas migration and accumulation are controlled by structure characteristics:① The control action of oil source fault of oil source condition. The study area has the characteristics of multiple for hydrocarbon. Oil source fault arc mainly distributed in the front of the nose structure and flank with big slip and long stretch. According to the source relations of butt joint,it has been clear about the control of distribution regularity of different oil source fault; ②The control action of tectonic fracture system of hydrocarbon migration accumulation. The height of the upthrow side of oil layer is controlled by the size of oil source fault slip, namely, the bigger the butt "window", the greater the oil height. In addition, the basin tilting fault communicate oil source, antithetic fault is advantageous to the oil and gas enrichment. ③ Nose-like structure background and favorable structural trap are conducive to oil and gas enrichment. The Luojia nose-like structure is the favorable places for oil-gas migration and accumulation, plunging Bonan subsag in the north.2) Multi-factor matching controls the reservoir types:different reservoir types are controlled by tectonic fracture, formation denudation and sedimentary reservoir. The primary formation of braided river sedimentary is composite reservoir of strata, tectonic and lithologic. The primary formation of Alluvial fan sedimentary is lithology-structure and structural reservoir, which controlled by the tectonic; The primary formation of Delta formation sedimentary is lithologic-structural、structural reservoir which controlled by tectonic; Steep slope fan mainly form fan lithologic reservoir of lateral root barrier, or updip pinchout lithologic reservoir.3) Reservoir property controls the difference of oil content difference:on the one hand, reservoir control the dominant migration pathway, on the other hand control favorable oil and gas accumulation zone. Thick layer of sand body is also beneficial to oil and gas source of far lateral conducting. For the accumulation of oil and gas in the structural high provides the necessary conditions.oil source docking relations, the hydrocarbon accumulation of reservoirs are controlled by the development degree and physical property of reservoir. High pressure area routine test presents the characteristics of "If not reservoir, that is, dry layer" Statistics and fitting analysis of thick reservoir permeability is higher than 3md test can achieve industrial capacity. From the length ways point of view, the reservoir in the Lower Submember of the Fourth Member of Shahejie Formation is the acid diagenetic environment. Sand bodies buried depth is more shallow, physical property, the better; When the buried depth of more than 4200 meters, even if is development of secondary pore, but physical property totality bad.7. Four exploration results have been achieved in practical application:1) Exploration directions have been defined. On the basis of comprehensive evaluation, potential directions have been clarified, several favorable exploration areas have been discovered, which greatly enlarged the exploration space of Sha 4 lower submember. Areas that were once thought with poor developed source-related fault can be oil-bearing, same with the low-lying zones.2) Success rate of exploration wells have been improved. Based on comprehensive evaluation,12 wells are suggested,7 of which have been adopted, such as the wells Yi178, Yi 179. Recently, the wells Yi178, Yi179, Yi185, Yi186, Yi193 have been finished drilling, all of which are interpreted to be oil-bearing.3) Control reserves have been reported. The oil-geology reserves of Sha 4 lower submember of the well Luo68 area are reported as 720.05*104 tons, the well Yi178 are 599.99*104 tons; 4) New breaks have been made:① Breaking the forbidden zone of explorations:Former explorations aiming on red layer of Sha 4 lower submember placed more emphasis on exploring the source-related faults and structural traps. However, the research shows the northern area, the source-related faults of which once were thought to be poor developed, can be oil-bearing, low-lying areas around Luojia nose structure show certain exploring potentials.Recent wells such as the Yi185, Yi193 are interpreted to be oil-bearing, this helps to expands the oil-bearing area northward, as well as the stratum vertically (Both 1,2 sandstone units met oil-bearing layers); ② A new set of source rock has been discovered—Sha 4 lower submember source rock, which changed the traditional opinion. The reservoir of well Yi185 is defined as self-generated and self-stored by geochemistry analysis; ③ New reservoir type has been identified:litho-structural and structure-lithological reservoir types formed by shallow water delta. Based on the analysis of reservoir formation, new reservoir type which included in the source rock of Sha 4 lower submember is developed on the downthrown side of the northern steep slope— lithologic reservoir. The results of the study effectively guided the red layer exploration in the Bonan Sub-sag. The Sub-sags of the Bohai Bay basin have similar geological conditions of formation of reservoir. The studying results can offer guidance for researches on the Bohai Bay fault basins sedimentary reservoirs and source rock. |
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