Pore structure differences of the extra-low permeability sandstone reservoirs and the causes of low resistivity oil layers: A case study of Block Yanwumao in the middle of Ordos Basin, NW China

The influence of pore structure difference on rock electrical characteristics of reservoir and oil reservoir was analyzed taking Triassic Chang 6 reservoir in Block Yanwumao in the middle of Ordos Basin as an example. The relationship between the pore structure difference and the low resistivity oil layer was revealed and demonstrated through core observation, lab experiments, geological research, well log interpretation and trial production etc. The results show that there were two kinds of oil layers in Chang 6 oil layer set, normal oil layer and low resistivity oil layer in the region, corresponding to two types of pore structures, pore type mono-medium and micro-fracture-pore type double-medium; the development of micro-fracture changed greatly the micro-pore structure of the reservoir, and the pore structure difference had an important influence on the rock electrical characteristics of the extra-low permeability sandstone reservoir and oil reservoir; the normal oil layers had obvious characteristics of pore-type mono-medium, and were concentrated in Chang 61, Chang 6232 and Chang 62; the low resistivity oil layers had obvious characteristics of micro-fracture-pore type double-medium, which were mainly distributed in Chang 612 and Chang 63. The mud filtrate penetrated deep into the oil layers along the micro-cracks, leading to sharp reduction of resistivity, and thus low resistivity of the oil layer; the low resistivity oil layers had better storage capacity and higher productivity than the normal oil layers.

Characteristics of Chang 2~1 Low PermeabilitySandstone Reservoir in Shunning Oil Field

Characteristics of Chang 21 low permeability sandstone reservoir of Shunning oil field are analyzed and evaluated based on the data of well logging and experiment. The result shows that 1) the Chang 21 low permeability reservoir belongs to the classification of middle-to-fine sized feldspar sandstone, with its components being low in ma- turity, deposited in distributary rivers in the front of the delta; 2) the reservoir is obviously dominated by a low or a very low permeability with a linear variation tendency different from that of the ultra-low permeability reservoir; 3) the spa- tial variation in lithology and physical properties of the reservoir are controlled by the sedimentary facies zones, and 4) the physical property of the reservoir is significantly influenced by clastic constituents and their structure, and the con- stituent of cement materials and their content. The result also shows that the diagenesis action of the reservoir is quite strong in which dissolution greatly modified the reservoir In addition, the inter-granular dissolved pores are the mainly developed ones and the micro-structure is dominated by the combination of middle-to-large sized pores with fine-to-coarse throats. Finally, the radius of the throats is in good exponential correlation with permeability and the seepage capacity comes from those large sized throats.

吴旗探区长6^1储层构造裂缝特征及分布规律

吴旗地区主力油层为上三叠统延长组长6^1，属于特低渗储层。根据研究区露头地层、钻井岩心及岩石薄片实物资料，同时利用古地磁岩心定向方法，对研究区构造裂缝进行了详细的定性一定量化观测描述和统计。研究认为，吴旗地区长6^1储层主要发育NE向和NW向高角度的区域性共轭构造裂缝系统。储层段岩心构造裂缝密度为0．4～1．90条／m，构造裂缝以偏张性为主，充填较为严重（约占57％），无效裂缝居多（约占68％）。在此基础上，以构造裂缝观测值为依据，通过构造应力场有限元数值模拟，综合岩石破裂法和能量法，建立了裂缝预测数学模型，对长6^1储层构造裂缝分布和发育规律进行了定量预测研究，指出了裂缝发育区、次发育区和不发育区；同时，预测了张、剪裂缝发育方位，为吴旗地区低渗透储层石油进一步勘探部署和区块合理开发提供了地质依据。

志丹油田洛河区块长6^1低阻油层测井识别方法研究

陕北志丹油田洛河区块长6^1油层作为该区块的主力油层,区域内低阻油藏较为发育,但由于低阻成因机理较为复杂、表现形式多样,常规识别方法符合率较低。通过对岩心化验分析资料及测井资料的对比分析,认为束缚水饱和度高、地层水矿化度高是形成低电阻率油层的主要油藏背景条件。根据淡水钻井液侵入对油层、水层深浅电阻率影响的差异,运用侵入因子识别法和横向对比识别法可有效提高对低阻油藏的识别符合率。

鄂尔多斯盆地志丹、安塞地区长6低阻油层成因机理及识别方法

陕北中生界延长组地层中低阻油层较发育,其分布广泛,资源潜力巨大。由于其成因机理复杂,表现形式多样,传统的方法难以识别,因此,结合油田生产实践,以录井、测井、实验分析及试油试采等资料为依据,并以主力产油层组为研究对象,在综合地质研究的基础上,对志丹、安塞地区长6低阻油层的成因机理进行了研究,并探讨了对其进行识别评价的方法。研究结果表明:孔隙结构复杂、黏土附加导电能力以及高地层水矿化度是研究区长6低阻油层主要的成因机理;采用交会图法、测井曲线重叠法及邻近水层对比法可以提高对低阻油层的识别准确度。

鄂尔多斯盆地铁边城地区长6层油层低阻特征及影响因素分析

鄂尔多斯盆地姬塬油田铁边城地区三叠系延长组长4+5、长6区块属于典型的陆源碎屑沉积的低渗—特低渗透致密砂岩储层,受后期强烈的成岩作用改造,是盆地内建产重要地区之一。通过岩心观察、扫描电镜观察、砂岩薄片鉴定、高压压汞分析,以及核磁共振测试等多种技术手段,还结合钻井、测井、录井、试油试采及储层特征等资料,在通过与全区横向对比的基础上,综合分析了该区块低阻油层的特征及影响因素。

特低渗砂岩储集层孔隙结构差异与低电阻率油层成因——以鄂尔多斯盆地中部烟雾峁区块为例

以鄂尔多斯盆地中部烟雾峁区块三叠系延长组长6油层组为例,通过岩心观察、实验分析、地质研究、测井解释、试采验证等,探讨孔隙结构差异对储集层及油层岩石电学特征的影响,论证并揭示了孔隙结构差异与低电阻率油层的成因关系。区内长6油层具有正常电阻率油层和低电阻率油层两种表现形式,发育孔隙型单孔介质和微裂缝-孔隙型双孔介质两种结构类型;微裂缝的发育极大地改变了储集层的微观孔隙结构,孔隙结构差异则对特低渗砂岩储集层及其油层的岩石电学特征产生了重要影响;正常电阻率油层具有明显的孔隙型单孔介质特征,主要集中于长61亚油层组和长622、长623小层,低电阻率油层具有明显的微裂缝-孔隙型双孔介质特征,主要集中于长621小层和长63亚油层组。钻井液沿微裂缝对油层形成了超深侵入,导致电阻率大幅降低及油层的低电阻率现象,低电阻率油层比正常电阻率油层具有更好的储产能力。