渤海湾盆地南堡凹陷南部深层沙一段优质碎屑岩储层特征及成因机制

Characteristics and genetic mechanism of high-quality clastic reservoirs in the 1st Member,Paleogene Shahejie Formation in the southern Nanpu Sag,Bohai Bay Basin

利用岩芯、薄片、扫描电镜和分析化验等资料,对南堡凹陷南部深层古近系沙一段(沙河街组一段)优质碎屑岩储层特征进行了分析并揭示其成因机制。研究表明:(1)南堡凹陷南部沙一段优质碎屑岩储层属于低孔中渗储层,发育原生孔隙、次生孔隙和微裂缝;(2)沙一段沉积期处于强水动力沉积环境,辫状河三角洲前缘水下分流河道砂体普遍发育,岩性主要为中粗砂岩、含砾砂岩和砂砾岩等粗岩性,岩石类型主要为岩屑长石砂岩,石英、岩浆岩岩屑等刚性组分含量高,储层在强压实作用下仍保留了局部原生残余粒间孔,部分刚性组分强压条件下形成微裂缝;(3)沙一段与东三段为区域不整合接触,大气水的酸性流体在成岩初期以及烃源岩成熟之后产生的有机酸在成岩后期通过油源断裂和不整合面进入储层,造成长石、易溶碎屑等组分溶蚀形成次生孔隙,溶蚀作用不仅增加孔隙空间,而且扩大喉道宽度。结论认为:粗岩性和高刚性颗粒含量为原生孔隙的保存提供了物质基础;油源断层、不整合面为酸性流体的进入提供路径,促进了次生孔隙发育。

Objectives:It is generally considered that the lower limit of the effective reservoir depth of clastic reservoirs in the Nanpu Sag of the Bohai Bay Basin is 4 km.If deeper than 4 km,that the deep clastic reservoirs would be no longer have industrial oil and gas production capacity.However,the Well PG2 in the southern Nanpu Sag encountered high-quality clastic reservoirs in the 1st Member of Paleogene Shahejie Formation (Es1) to a depth of4.2 km and obtained a high-production oil&gas flow.In order to reveal the characteristics and genesis of this reservoirs,and reduce the exploration risk of deep oil and gas layers,we discussed its characteristics and genetic mechanism.Methods:To investigate the characteristics and genetic mechanism of the Es1reservoirs,thin sections and scanning electron microscopy (SEM) images were obtained from the Petro China Jidong Oilfield Company.Reservoir physical properties,including porosity and permeability,were evaluated from 126 core samples selected from 5 boreholes;the samples were tested with a RecCore-04 Nuclear Magnetic Resonance Rock Spectrometer at room temperature (20℃).The grain size,diagenetic features,and porosity were evaluated from 127 samples acquired from 3 boreholes,and thin sections were analysed under a polarizing microscope to highlight the reservoir spaces;the thin sections were dyed with blue epoxy.A total of 27 samples from 1 boreholes were selected,and the samples were tested using a QUANTA 200 scanning electron microscope with an accelerating voltage of 20 kV and a current of 50~100 pA.Results:Research shows that the type of reservoirs deeper than 4 km in south of Nanpu Sag are low porosity and medium permeability,and reservoirs space incudes primary pores,secondary pores and micro-fractures.The sedimentary period of Es1was in a strong hydrodynamic sedimentary environment.The braided river delta front divergent channel sand bodies were widely developed.The lithology is mainly coarse lithology such as medium—coarse sandstone,conglomerate sandstone and gravel.Rock type is mainly lithic feldspar sandstone.High content of rigid components such as quartz and magmatic rock cuttings.The reservoir still retains some of the original residual intergranular pores under strong compaction,and some rigid components form micro-fractures.There is regional unconformity contact between Es1and Ed3.The organic acid produced by the acidic fluid of atmospheric water in the early stage of diagenesis and after the source rock matures enters the reservoir through the oil source fracture and unconformity surface in the late diagenesis,resulting in feldspar and soluble debris dissolves to form secondary pores.The dissolution not only increases the pore space,but also enlarges the throat width.Conclusions:It is concluded that coarse lithology,high content of rigid components provide the material basis for the preservation of primary pores.Oil source faults and unconformities provide pathways for the entry of acidic fluids and promote the development of secondary pores.And the dissolution effect is better than the reservoir capacity for increasing the reservoir permeability.This study provides references for the evaluation of high-quality reservoirs and benefit exploration in the Bohai Bay Basin.