Coal-forming environment of Lower Cretaceous in Huhehu depression was analyzed from structure and filling sequence.Lower Cretaceous of Huhehu depression went through two tectonic evolution stages,i.e.extensional faulted depression and post-fault thermal subsiding fault sag.Extensional faulted depression stage can be divided into initial tension crack stage,strong tension crack stage,rapid subsidence stage of fault sag;post fault thermal subsidence stage can be divided into fault-sag transition subsiding stage and fault-sag subsidence stage.According to the association characteristics of filling sequence of faulted basin,the strata of Lower Cretaceous in Huhehu depression were identified from bottom to top as coarse clastic alluvium member(Fm) at the base,lacustrine fine-grained deposit member(Lm),coal-bearing clastic rock member and lacustrine member(Cm-Lm),lacustrine fine-grained detrital deposit member(Lm),coal-beating clastic rock member and lacustrine fine deposit member(Cm-Lm) and coal-beating clastic rock member(Cm).Combining the structure evolution and filling sequence,this paper discussed the coal-forming environment of the second member of Nantun Formation,the second member of Damoguaihe Formation and Yimin Formation.During sedimentation of the second member of Nantun Formation,coal-bearing strata were mainly developed in the lake shore(where swamp was developed in long term and stably in faulted basin) and the abandoned braided river delta.During sedimentation of the second member of Damoguaihe Formation,fault-depressed shallow water basin,the northern coal accumulation was mainly developed in fan delta sand body,and the southern coal accumulation was mainly developed in the lake shore where swamp was developed in long term.During sedimentation of Yimin Formation,depression was filled,and the shallow marshes became major coal-bearing places.

运用断层生长指数联合断层垂直活动速率方法,判断同生断裂活动时期为铜钵庙组～南屯组沉积时期、伊敏组时期和青元岗组沉积时期,同时,根据断层伸展率和伸展速率的研究得出,同生断裂在南屯组沉积时期变形强度较大,为断层活动的主要关键时期.同生断裂经历了从小到大、从新到老的过程,总体表现为纵向上断裂活动多期性,且由深到浅断裂活动强度逐渐减小的特点,横向上不同位置走向具有分段明显,并利用断裂分段生长点确定了断裂传递带的位置.在此基础上,总结了乌尔逊-贝尔凹陷南屯组主要发育2种控砂模式——同生断裂下降盘控砂和分段生长点控砂模式.同生断层控砂模式与地层沉积演化序列相一致,不同的沉积演化决定了断层控砂类型.南一段主要以断层下降盘和分段生长点控砂为主,南二段表现为断层分段生长点控砂,其中断层下降盘控砂发育有限,主要发育在南一段巴彦塔拉构造带处,断层分段生长点控砂在盆地发育较广泛,是油气聚集的有利场所.

为了实现特低渗透油藏“难采、难注”储量的有效动用,探索提高采收率的新方法和新技术,在某油田特低渗透储层开展了大规模压裂技术试验.优选有效厚度大、单井储量控制大、初期产量较高、产量递减快的6口井,采用“隔井错层压裂”方式构建压裂矩形井网.通过提高压裂液量和压裂加砂量,增大压裂波及体积,改善储层渗流条件.试验表明“定向、定量造长缝”的大规模压裂技术能够有效改善储层渗流条件,全面实现各小层动用,获得较高的初期产能.同时,合理有效注水能够保障大规模压裂后油井持续高产稳产.

Huhehu Depression is a key exploration area in the east grabens which is located in the southeast of Hailar Basin.The depression is a half-graben rift subsidence structure,characterized by it being faulted in the east and overlapped in the west,and its area is 2 500 km2.The 2nd member of the Nantun Formation in Huhehu Depression has developed the coal measures buried deeply whose strata formed by a set of gray siltstone and dark thick mudstone which mixed with black coal,gray fine sandstone and grey conglomerate.And the rock layers developed a series of multilayer and thicker coal seam,its reflectance of vitrinite is between 0.8%～1.5%,and the coal rank between gas coal-coking coal stage,so the coal measures have a better hydrocarbon potential.Also,because of the weaker later tectonic movement,the coal measures contain rich oil-gas(CBM) resources.This paper mainly analyzes the characteristic of the stratigraphic framework,coal accumulation environments and coal accumulation laws in the case of the coal measures of the 2nd member in the Nantun Formation to guide the petroleum exploration in Huhehu Depression in the future. Based on the theory and method of sequence stratigraphy,according to the drilling,well logging and seismic data,the sequence interfaces(the top interface SB4,the bottom interface SB3) and system tract interfaces(defined by the initial and maximum flooding surfaces) of the 2nd member in the Nantun Formation are identified.Finally,the sequence stratigraphic framework is established by the comprehensive analysis of the tectonic evolution,paleontological features,paleoclimate changes and lake level variations.Also,the main controlling factors on sequence formation are analyzed briefly. At present,there are 11 wildcat wells in the study area.Through the analysis of the single well facies and sections of correlation of the stratigraphic,combined with other single factor maps,such as the stratum isopach maps,sandstone isopach maps,sandstone content isoline maps,dark mudstone,carbonaceous shale and coal layers thickness isopach maps,with the system tracts as the units,the author has described the sedimentary system distribution and temporal-spatial evolution characteristics within the sequence stratigraphic framework. Through comprehensive analysis of sedimentary evolution and correlation of the sequence stratigraghy,a study of coal layers development features and the patterns of coal accumulation is carried out with the application of basic theory and method of the coalfield geology.The 2nd member in the Nantun Formation of Huhehu Depression which was in the faults extension period,characterized by the low-amplitude,weak and pulsative vibration of the tectonic subsidence,During this period,it experienced an infilling evolution process of water in withdrawal,and the lacustrine-braided river delta sedimentary systems developed dominantly.In the early stage of base-level falling,the increasing rate of accommodation space was consistent with the supply rate of sedimentary source,and the lacustrine areas continuously existed in the center of the depression,so the peat swamp formed stably and accumulated coal in the lacustrine basin margin of the depression;With the base level rising up,the groundwater level rise regionally,the 2nd member of the Nantun Formation developed shore-shallow lacustrine swamp coal-forming environments.Because of the peat beds were buried and preserved rapidly by the highstand system tract sediments,the coal-forming conditions of transgressive system tract were favourable;Lastly,in the highstand system tract sedimentary period also developed the coal accumulation environment,as a result of progradation of braided river delta,a lot of peat layers were buried rapidly in the braided river delta plain.Accordingly,controlled by the lake level changing system and the relatively steady tectonic movement,the major coal accumulation environments are interdistributary estuary swamp of the delta plain and shore-shallow lacustrine swamp. As mentioned above,the study area was controlled by the rates of the tectonic subsidence and the peat accumulation every system tract developed coal-accumulation sedimentary conditions.However,the degrees of development and distribution ranges of the coal layers are significantly different between the different system tracts.The results indicate that the coal-accumulation mainly developed in the later period of lowstand system tract and transgressive system tract.In vertical directions,the coal layers mainly formed in transgressive system tract where the water started to expand.During this period,a large area and long term lacustrine-braided river delta sedimentary systems were peaty and swampy largely in shore-shallow lacustrine zones where the water was shallow.Under such circumstances,because of the increasing rate of the accommodation space matched balanced with the sedimentary accumulation rate,the stratum was very conducive to form series thick and stable coal layers.But the coal accumulation was the weakest in the highstand system tract in which the distribution of formed coal layers was thin and more localized. Controlled by the tectonic subsidence and sedimentary systems,the coal accumulation of each system tract shows obvious zonations and situations that South-east of the Depression is strong,while Northeast is weak.The coal-rich units are in accordance with the distribution of the lacustrine-braided river delta sedimentary facies,mainly distribute between lacustrine basin margin and the subsidence center,and near to the lacustrine basin margin.However,it was not conducive to develop peat swamp in the deeper lake water body,the coal layers became thin even pinchout from the braided river delta plain to deep lake.

The petroleum exploration in the Huhehu Depression of Hailar Basin has become a new area for searching for reserves in the Daqing Oilfield, but no great breakthrough has been made owing to the lower study degree. According to the seismic reflection termination relationship and the regional geological background, the Cretaceous in the Huhehu Depression can be divided into eight seismic sequences, Sq1-Sq8. According to the internal reflection texture and external geometric configuration and in combination with the parameters of amplitude and continuity, eight seismic facies are identified in the target sequences of the study area, including parallel sheet, sub-parallel sheet, prograding, filling, wedge shaped, moundy, chaotic and wave-hummocky seismic facies. And their characteristics are described in detail. These seismic facies show differentiation in horizontal distribution and are successive and evolutionary vertically. Seismic facies are transformed into sedimentary facies through a comprehensive interpretation. The seismic - sedimentary facies model of each sequence is established according to the lacustrine sedimentary theory. The results show that the target sequences develop the sedimentary systems of alluvial fan, fan delta, braided river delta, lake and sublacustrine fan.

Sedimentary facies types,sedimentary characteristics and distribution law of the Nantun Formation of the Lower Cretacic in the Huhehu depression are analyzed based on the core,well log curve,analysis assay,seismic and so on. The research indicates that the Nantun Formation developed fan delta,braided river delta,sublacustrine fan and lacustrine fan. This paper expounds in detail sedimentary characteristics and identification mark of fan delta,braided river delta and sublacustrine fan in cause,source supply,development position,sedimentary characteristics and seismic reflection characteristic. Fan delta is distributed mainly in the northwestern gentle slope zone and southeastern steep slope zone of the 1st Member of Nantun Formation; braided river delta in the 2nd Member of Nantun Formation and in the northern slope zone of Nanyi section; sublacustrine fan developed in deep lake-half deep lake facies. This paper points out that southern sedimentary sand of the Huhehu depression is the favorable exploration region,in which fan delta and braided river delta front subphase zone is the important hydrocarbon accumulation.

Based on the theory of sequence stratigraphy,sequence stratigraphy framework of Nantun group in Huhehu Depression is established in this paper by comprehensively research of cores,logging data and seismic data.Nantu group in Huhehu Depression is divided into 2 3rd-order sequences:SQ1 sequence is equal to the first stage of Nantu group,and SQ2 is equal to the second stage of Nantun group.Neither sequences develops completely,which is composed of low stand system,transgressive system tract and high stand system.According to the characteristics of divide sequence development of target strate in researched area,and based on the division and comparison of sequence formation unit,this paper studies on sedimentary types and system,determined that fan delta,braided river delta,sublacustrine,lake and so on are very development.SQ1 sequence mainly develops braided river delta in the north of basin,and fan delta in the south of basin.SQ1 sequence mainly develops braided river delta in gentle slope belt,and fan delta in steep slope in local area.Compared with SQ1 sequence,SQ2 develops swamp facies.Comprehensive analysis of sedimentary characteristics and evolution law,this paper points out that sedimentary sand bodies in the south of Huhehu Depression is the favourable exploring area.

The formation and evolution of fault played important roles on the accumulated reservoir of hydrocarbon. The characteristics, evolution of fault and the relation between reservoir of Huhehu depression were analyzed in this article. The results show that, with the T22 reflection horizon for boundary, Huhehu depression could be divided into three main structural layers, the lower extensional fault-depression layer, the middle faulting-sinking layer and the upper dawn-warping layer, in which fault aplasia in the upper dawn-warping layer. There have been two faulted systems which were developed in the longitudinal direction, the spreading direction of the lower faulted system mainly with NEE, NE and NNE direction and the upper faulted system mainly with NNE and NNW direction. There are four types of faults in this area, including early extensional faults, middle strike-slip faults, early extensional and middle strike-slip faults, early extensional and middle strike-slip and later reverse fault. The main formation periods of faults are in the late depositional periods of Nantun, Yimin and Qingyuangang Formations. The formation and evolution of fault influenced the distribution and thermal evolution of source rock, formed multi-type traps and provided a good migration channel for the vertical migration of hydrocarbon. So the main petroleum generating sub sag and the areas among controlling-depression fault nearby the main petroleum generating sub sag are beneficial to hydrocarbon enrichment.

Applying the principles and methods of high-resolution sequence stratigraphy,and combining with the data of exploration wells and development wells in encrypted network of wells,the authors identified 41 short-term base-level cycles of 7 types,which were formed under the conditions of deeping upward,shallowing upward,symmetrical in different accommodation space in Fuyu oil layer of Chao 1-Chao gas 3 blocks,and makes detailed analysis on the genesis and characteristics based on the sedimentary dynamics.According to the structural feature and stacking pattern of short-term base-level cycle,the authors established seven medium-term base-level cycles and identified two long-term base-level hemicycles by accompanying local indicator layers,and thus established the high-resolution sequence stratigraphic framework.In the study area,sand body is generally absent in the long-term base-level fall hemicycles,but in the early long-term base-level rise hemicycles,the thickness and effective thickness of sand of river-facies are significant,ratios of sand to mud are high,oiliness is good,and with the long-term base-level rising,the sedimentary environment turned to the delta system,the shore lake and the shallow lake,sand and oiliness gradually decrease.Moreover,each medium-term base-level,sand and its oilness in the transform zone from subsidence to elevation developed relatively best in the whole medium-term base-level cycle.