贝尔凹陷下白垩统层序格架下砂体分布特征

以层序地层学理论为指导，综合地震、测井、岩芯以及古生物等资料，对贝尔凹陷下白垩统层序格架下砂体的分布特征进行研究。研究表明:在三级层序格架下，不同沉积期沉积体系的空间展布和配置关系有明显差异，SQ_A层序为初始裂陷型层序，主要发育冲积扇、近岸水下扇等粗碎屑沉积体系；SQ_B～SQ_C层序为伸展裂陷型层序，发育近岸水下扇、湖底扇、扇三角洲和半深湖相等沉积体系；SQ_D～SQ_F层序为裂陷收敛型层序，发育三角洲、滨浅湖和半深湖以及河流、沼泽相等沉积体系。低位域砂体为岩性圈闭的形成提供有利条件，其时空展布主要受幕式构造运动及其产生的构造坡折带的类型和样式控制。在垂向上，构造坡折带类型控制不同类型低位域砂体的形成和分布，其中断崖型坡折带形成近岸水下扇，断阶型坡折带形成浊积扇以及断坡型坡折带形成扇三角洲砂体；在平面上，构造坡折带平面样式控制低位域砂体的空间展布特征，尤其是砂体的形态及规模受断层调节带控制。受同沉积断层组合样式的控制，平面上主要形成断弯型、断接型、断沟型以及交叉型等4种基本砂体分布类型以及断弯-交叉型、断弯-对接型、断弯-断沟型、对接-交叉型、对接-断沟型和交叉-断沟型等6种复合砂体分布类型。低位域砂体在洼槽区相互叠置、连片分布，为各类岩性-地层圈闭的形成创造条件。

Distribution of Sandbodies in a Sequence Stratigraphic Framework in Lower Cretaceous of Beier Depression

Hailaer Basin, featured by frequent structural movement, simple depositional cycle, multi-and near-sources, coarse clastic, narrow-sedimentary facies and more fans, is Mesozoic Early Cretaceous fault depressed lacustrine basin developed on the Hercynian geosyncline folded base in northeast China. After its formation, Hailaer Basin had gone through three tectonic phases, including rift initiation,  rift extension  and  post-rift episodes, and each phase has its own depositional system. Based on the theory of sequence stratigraphy and used all data such as seismic, well log, core analysis and palaeophyte, this paper focuses on sandbodies distribution in a sequence stratigraphic framework in Lower Cretaceous of Beier depression. Studies show that there are three second-order sequence correspond to three tectonic phases, including rift initiation sequence SQ_Ⅰ(Tongbomiao group K₁t), rift extension sequence SQ_Ⅱ(Nantun group K₁n) as well as post- rift sequence SQ_Ⅲ(Damoguaihe group K₁d), and six third-order sequence including SQ_A(Tongbomiao Group K₁t), SQ_B(Nanyi member K₁n₁), SQ_C(Nan’er member K₁n₂), SQ_D(Dayi member K₁d₁), SQ_E(Daer member K₁d₂) and SQ_F(Yimin Group K₁y) as well as fourteen systems tracts. In the third-order sequence stratigraphic framework, the spatial distributions and sedimentary assemblages of depositional system are varying significantly in different depositional stage. In the depositional period of sequence SQ_A(formed in the rift initiation episode), which developed the coarse clastic sedimentary assemblage as alluvial fan and subaqueous fan. During the depositing period of sequence SQ_B～SQ_C (formed in the rift extension episode), which developed the sedimentary assemblage as subaqueous fan, turbidite fan, fan delta and semi-deep. While during the depositing period of sequence SQ_D ~SQ_F(formed in post-rift episode), which developed the sedimentary assemblage as delta, shore-shallow lacustrine and semi-deep, as well as fluvial and paludal facies.
Lowstand system tracts sandbodies were mostly distributed in rift extension sequence SQ_Ⅱwith favorable reservoir conditions. Meanwhile, the above thick claystone in transgressive and highstand system tracts act as cap rocks at the same time. So they can firm favorable reservoir-seal assemblage and lithologic-stratigraphic traps. Studies also show that lowstand system tracts sandbodies are mainly distributed in the depositing period of sequence SQ_A～SQ_C, while few in SQ_D～SQ_F. The main types of sandbodies are braided river channel, subaqueous fan, turbidite fan, fan delta and so on, which are featured by muti-layers with thick single layers. Controlled by multi sources, lowstand system tracts sandbodies had superposed and jointed in most of the depression.
The sandbodies controlling factors are very complicate in these kinds of basins. From the aspects of analyzing the function of structure controlling depositional system, synsedimentary structures often play an important part in sandbodies deposition and its distribution. Especially, lowstand system tracts sandbodies, of which spatial distribution are mainly influenced by episodic tectonic movements  and the types of structural slope break zone formed by many different level and types of faults, is the most important reservoirs for lithologic traps. The formation and distribution of lowstand system tracts sandbodies is mainly controlled by the types of structural slope break zone vertically. Three types of structural slope break zone developed on the basin margin,  resulting in three types of depositional systems. Nearshore subaqueous fan system resulted from faulted scarp type break in-slope; slope fans system developed on faulted terrace type break in-slope; fan-deltas and sub-lake fan system developed on steep slope type break in-slope. While in plane, sandbodies, featured by thin thickness, small range of extension, a rapid horizontal variation and along fault strike in bands or apron shape, are controlled by the style of faulted slope-break zone. Especially,  the shape and scale of sandbodies is controlled by fault-accommodation zones. Four basic types fault-accommodation zones including fault-bend, fault-connect, fault-valley and fault-intersect, and six complex types fault-accommodation zones including fault-bend and fault-intersect, fault-bend and fault-connect, fault-bend and fault-valley, fault-connect and fault-intersect, fault-connect and fault-valley, and fault-intersect and fault-valley are concluded by genesis. As a result, different kinds of sandbodies tend to be superposed and jointed in middle sag, which are the material basis of lithologic-stratigraphic traps-forming.
In system tracts sequence stratigraphy framework, controlled by fault-accommodation zones and their combinations, sandbodies have various  scales and shapes and show striped distribution along fault strike in Beier depression. During the depositing period of lowstand system tracts of sequence SQ_B, sedimentary facies of this area is shore-shallow lacustrine to semi-deep lake sediments. The lowstand system tracts of Nanyi member mainly develop fan delta, subaqueous fan and turbidite fan. Especially, in southwestern Beixi sub-depression, fault-intersect type mostly developed in steep slope for fault terrace, while fault-bend and fault-connect types in gentle slope for many consequent faults. For small deposition area and shallow water, this area has many kinds of lowstand system tracts sandbodies, which are controlled by fault-accommodation zones and show striped distribution along the long axis direction of Beier depression.