Stratal Slice Recognition of Thin Shallow-Water Delta Sandbodies in the Songliao Basin

Objective China＇s petroleum exploration has entered a new stage of finding deeply buried thin sandbodies lbr the abundant oil resources they contain. Here thin sandbodies refer to those less than 10 m in thickness, or even less than 1-2 m. It is difficult to depict thin-layer sandbodies of different genetic types using conventional core, well logging and seismic data due to their limited vertical resolution in petroliferous basins. However, seismic sedimentology provides a new research method especially tbr thin sandbody interpretation, i.e., validating interpreted sedimentary sandbodies from 3D seismic data based on horizontal resolution, stratal slice and seismic geomorphology interpretation. At present, a series of studies on seismic sedimentology in North America marine basins and elsewhere have been completed successfully and are relevant to the exploration and development of oil and gas fields.

The influence of water level changes on sand bodies at river-dominated delta fronts:The Gubei Sag,Bohai Bay Basin

Changes in water level are one of the important factors controlling the constructive characteristics of deltas.The paper studies the influence of water level changes on sand bodies in the third member of the Shahejie Formation(Es3)on the gentle southern slope of the Gubei Sag,Bohai Bay Basin and draw some conclusions that,for complex sand bodies,with the increase in water level the distributary channels bifurcate frequently,from a simple branching shape to a network shape along with the increase in the development of crevasse splays,mouth bars and sheet sands.For single sand bodies,with an increase in water level in the slope zone of the lake basin close to the source area,the superimposition style transitioned from vertical cutting-stacking and lateral isolation to vertical stitching,isolation and lateral stitching.However,in the central zone of the lake basin far from the source area,the superimposition style transitioned from vertical stitching and lateral stitching to vertical isolation and lateral isolation.When water level stays stable,the greater the distance from the source area the greater the disaggregation ratio of a single sand body.At the same distance from the source area,higher water level tends to result in greater disaggregation ratio of a single sand body.

Sedimentary characteristics of distributive fluvial system in arid area: A case study of the Shule River distributive fluvial system, NW China

Based on detailed investigation of the modern sedimentation of the distributive fluvial system of Shule River and the data of unmanned aerial vehicle(UAV)aerial photography and satellite remote sensing,the sedimentary characteristics and differences of distributive fluvial system in arid areas are analyzed.By comparing the changes in slope,river morphology and sedimentary characteristics in different sections from the apex to the toe,the distributive fluvial system of Shule River can be divided into three facies belts:"proximal","middle"and"distal".The proximal belt has the largest slope and strongest hydrodynamic condition,mainly appears as large-scale braided river deposits;the fluvial bars in this belt are mainly composed of gravels,the gravels have good roundness and certain directionality,and are medium-large boulders,with low sand content;the main microfacies in this belt are braided channel and flood plain.The middle belt with slope smaller than the proximal belt,is mainly composed of braided bifurcating river deposits.Due to branching and infiltration,this belt has weaker hydrodynamic conditions,so some of the distributive rivers dry up,appearing as ephemeral rivers.This belt has small lenticular sandbodies,fine to medium gravels,higher sand content,and mainly braided channel,flood plain and aeolian dune microfacies.The distal belt has the smallest slope and flat terrain,where the river begins to transform from braided river to meandering river,the sediment is mainly sand.Due to the influence of slope,this belt has weaker erosion toward source and stronger lateral erosion,and point bars developing around the edge of the active lobes.In this belt,the river is completely meandering,and the main microfacies are braided channel,meandering channel,flood plain,aeolian dune,lake and swamp.

Sandbody architecture of the bar finger within shoal water delta front:Insights from the Lower Member of Minghuazhen Formation,Neogene,Bohai BZ25 Oilfield,Bohai Bay Basin,East China

Core, well logging and seismic data were used to investigate sandbody architectural characteristics within Lower Member of Minghuazhen Formation in Neogene, Bohai BZ25 Oilfield, and to analyze the sedimentary microfacies, distribution and internal architecture characteristics of the bar finger within shoal water delta front. The branched sand body within shoal water delta front is the bar finger, consisting of the mouth bar, distributary channel over bar, and levee. The distributary channel cuts through the mouth bar, and the thin levee covers the mouth bar which is located at both sides of distributary channel. The bar finger is commonly sinuous and its sinuosity increases basinward. The distributary channel changes from deeply incising the mouth bar to shallowly incising top of the mouth bar.The aspect ratio ranges from 25 to 50 and there is a double logarithmic linear positive relationship between the width and thickness for the bar finger, which is controlled by base-level changing in study area. For the bar finger, injection and production in the same distributary channel should be avoided during water flooding development. In addition, middle–upper distributary channel and undrilled mouth bar are focus of tapping remaining oil.

Lithofacies paleogeography restoration and its significance of Jurassic to Lower Cretaceous in southern margin of Junggar Basin,NW China

In view of the difficulties in the study of lithofacies paleogeography and the low reliability of the distribution range of sedimentary sand bodies in the prototype basin caused by less deep drilling, complex seismic imaging and low degree of exploration in the southern margin of Junggar Basin, NW China. A new method based on the source to sink idea was used to restore lithofacies paleogeography and predict glutenite distribution. In the restoration, apatite fission track age was used to define range and uplift time of macro-provenance;the range of provenance area and the migration process of lake shoreline were restored based on the quantitative relationship between gravel diameter and transportation distance, tectonic shortening and other geological parameters;drilling cores and field outcrop sedimentary structures were analyzed, and a series of maps of lithofacies paleogeographic evolution and distribution range of glutenite bodies were compiled. It is concluded that from Early Jurassic to Early Cretaceous, in the southern margin of Junggar Basin, the provenance area gradually expanded from south to north, the lake basin expanded, shrunk and expanded, and the paleoclimate changed from humid to drought to humid. The western section always had proximal fan delta deposits from the southern ancient Tianshan provenance developed, and in the middle and eastern sections, the provenance areas evolved from far source to near source, mainly river-delta, braided delta, fan delta and other sediments developed. The boundary between provenance areas of the western and middle sections is speculated to be Hongche fault zone. In an angle open to the northwest with the current basin edge line, the restored ancient lake shoreline controlled the heterogeneity of reservoirs in the delta plain belt and delta front belt on its both sides. The ancient lake shoreline, current stratigraphic denudation line and current basin margin line limit the types and scope of favorable reservoirs.This understanding provides an important geological basis for oil and gas exploration in the deep lower source-reservoir assemblage at the southern margin of Junggar Basin.

3-D Seismic Identification and Characterization of Ancient Channel Morphology

It is easy to identify ancient fluvial morphologic types by the outcrop,log and core data.However,the horizontal distribution and geometry of the channels can only be identified and predicted by relying on the 3-D seismic data.The 3-D seismic horizon slices,especially,can play an important role in the sandstone prediction of meandering rivers,distributary channels and low-sinuosity channels.Every microfacies unit,including main channels,such as sinuous or branching channels,levee,crevasse channels,ligule crevasse splay and floodplain etc.can be identified.Braided channel sandstones are planar tabular lateral-connected sandbodies and the distribution of thick main channel belts can only be identified from 3-D seismic data.As the braided sandstones are ubiquitous,their occurrence and distribution do not need to be predicted.Generally,the coal velocity is so low that it can create a strong amplitude reflection in coal strata.It consequently conceals the amplitude respondence to anastomosing channel sandstone which could be identified from 3-D seismic inversion data sometimes.Case studies of mud-rich low-sinuosity rivers identified with 3-D seismic data indicate that the scales and width-to-thickness ratio of such sandbodies are small,laterally unconnected,and generally occurred on distant or further parts of an alluvial fan under dry climate conditions.Sometimes extraction of seismic attributes of every reflection event along horizons is expected to maximize expression of the spatial evolutions of ancient channels.

Development characteristics and controlling factor analysis of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area, Bohai offshore basin

Based on a combination of high resolution 3-D seismic, drilling and well logging and core data, this study focuses on describing the depositional features of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area(HHKA), Bohai offshore basin and discussing the evolution and controlling factors of shallow water delta sandbody. An obvious meandering fluvial delta system developed in sequence 1(SQ1) of the Neogene in HHKA with thinner sandbody of distributary channels and poor development of mouth bar. The sequence texture obviously influences the vertical development and stacking pattern of sandbodies and controls the distribution of sandbodies in plain view as well. In shallow water lacustrine basins, relative topographic height difference leads to change of distribution of accommodation space, and sandbodies of distributary channels usually develop well in local low-lying areas where accommodation space increases. The delta is dominated by distributary channel sandbodies during the early period of base level rising. Sandbodies contact with each other in a lateral stacking pattern and are characterized by a fan shape in plain view. Distributary channels gradually narrow and tend to shift during the mid-late-period of base level rising, while the sandbodies are characterized by a net shape in plain view. During the period of base level slow falling, the multistory/multilateral channel sandbodies dominated the inner front of shallow-water delta and the delta sand dispersal distributes as a lobe shape.

A quantitative study of the scale and distribution of tight gas reservoirs in the Sulige gas field, Ordos Basin, northwest China

Gas and water distribution is discontinuous in tight gas reservoirs,and a quantitative understanding of the factors controlling the scale and distribution of effective reservoirs is important for natural gas exploration.We used geological and geophysical explanation results,dynamic and static well test data,interference well test and static pressure test to calculate the distribution and characteristics of tight gas reservoirs in the H_(8) Member of the Shihezi Formation,Sulige gas field,Ordos Basin,northwest China.Our evaluation system examines the scale,physical properties,gas-bearing properties,and other reservoir features,and results in classification of effective reservoirs into types Ⅰ,Ⅱ,and Ⅲ that differ greatly in size,porosity,permeability,and saturation.The average thickness,length,and width of type Ⅰ effective reservoirs are 2.89,808,and 598 m,respectively,and the porosity is>10.0%,permeability is>1010^(–3)µm^(2),and average gas saturation is>60%.Compared with conventional gas reservoirs,tight gas effective reservoirs are small-scale and have low gas saturation.Our results show that the scale of the sedimentary system controls the size of the dominant microfacies in which tight gas effective reservoirs form.The presence of different types of interbeds hinders the connectivity of effective sand body reservoirs.The gas source conditions and pore characteristics of the reservoirs control sand body gas filling and reservoir formation.The physical properties and structural nature of the reservoirs control gas–water separation and the gas contents of effective reservoirs.The results are beneficial for the understanding of gas reservoir distribution in the whole Ordos Basin and other similar basins worldwide.