Jiyang depression湖相页岩油地质及地化特征分析思路与方法——2015年新论文阅读笔记

  • Title: Geochemical and geological characteristics of the Es3L lacustrine shale in the Bonan sag, Bohai Bay Basin, China
  • Author: Min Wang, Ronald W.T. Wilkins, Guoqi Song, Linye Zhang, Xingyou Xu, Zheng Li, Guohui Chen
  • Abstract: Abstract
  • citation: Min Wang, Ronald W.T. Wilkins, Guoqi Song, Linye Zhang, Xingyou Xu, Zheng Li, Guohui Chen, Geochemical and geological characteristics of the Es3L lacustrine shale in the Bonan sag, Bohai Bay Basin, China, International Journal of Coal Geology, Volume 138, 15 January 2015, Pages 16-29, ISSN 0166-5162, http://dx.doi.org/10.1016/j.coal.2014.12.007. (http://www.sciencedirect.com/science/article/pii/S0166516214002705)

为了帮助我解决一些XRD方面的疑惑,石油大学的王教授把他最新发表的论文传给了我,记录一下收获。

这是针对Jiyang depression的湖相页岩(lacustrine shale)开展的研究,对shale oil而不是shale gas进行潜力评价,研究思路和方法对shale gas应该也很有启发。采用了 a Rock-Eval-VI instrument, a Panalytical X'Pert PRO Diffractometer with Cu Ka ra-diation (40 kV, 30 mA) and scanning speed of 2° 2 theta (θ) per minute, an FEI Quanta(High-resolution field emission scanning electron microscopy)三种仪器和手段.分析样品为数百块lacustrine shale core samples.除了电镜扫描外,其他两项分析手段基本是shale gas中的常用项目了,录井行业也能提供相应的服务。全篇文章的重点当然是Results and discussion部分了。

Organic matter

由于是shale oil,所以岩石热解分析结果中,关注的重心是S2(代表裂解烃)而不是shale gas中关注的S0(气态烃),其他方面应该是大同小异了。这里见到一个关于TOC的评价标准,是以前没有见过的。

  • TOC:Shales with less than 0.5, 0.5–1, 1–2, 2–4, and greater than 4 wt.% TOC can be categorized as poor, fair, good, very good, and excellent in quantity of organic matter (Peters and Cassa, 1994).

资料的详细来源是Peters, K., Cassa, M.R., 1994. Applied source rock geochemistry. In: Magoon, L.B., Dow,W.G. (Eds.), The Petroleum System—From Source to Trap. AAPG Memoir 60,pp. 93–120.

AAPG Memoir有权威性,就是年代稍微有点久远,不知道现在的专家会不会认可这个分类标准。

关于Types of organic matter,这方面录井做得工作不多,一般只是应用仪器自动提供的kerogen type,这方面研究机构/高校做的工作显然多得多,不仅分析kerogen type,还会分析具体的有机质成分。这里边最有意思的一项工作是plots of S2 versus TOC,用来分析kerogen type,方法来源于:Sanei, H., Petersen, H.I., Schovsbo, N.H., Jiang, C., Goodsite, M.E., 2014. Petrographic and geochemical composition of kerogen in the Furongian (U. Cambrian) Alum Shale,central Sweden: reflections on the petroleum generation potential. Int. J. Coal Geol.132, 158–169.

根据分析数据,采用S2与TOC的散点图,并做线性回归,得到了significant correlation coefficients of R2 = 0.8874的结论,从图上看也没什么问题,但是我有点困惑,不是热解分析一般都直接给出kerogen type这个结果的吗?为什么再用S2 versus TOC分析一下呢?不过做这个线性回归的另外一个分析结论很有吸引力,The S2–TOC regression lines intercept the TOC axis at 1.05 wt.%, which approximates the initial residual carbon (RC) or the initial inertinite content (dead carbon) of the sediments.参考文献中记录的该分析方法来源于Cornford, C., Gardner, P., Burgess, C., 1998. Geochemical truths in large data sets; I,Geochemical screening data. Org. Geochem. 29, 519–530.我厚着脸皮去骚扰了一下教授,教授解释说S2 versus TOC的截距代表有机质的吸附量,如果是shale gas,可以更换成S0 versus TOC,截距代表吸附气量,方法来源与大庆油田的一个研究成果:霍秋立,曾花森,张晓畅,付丽.松辽盆地北部青山口组一段有效烃源岩评价图版的建立及意义[J].石油学报,2012,03:379-384.,大致读了一下,文中提出了烃源岩中残留烃的概念(也许以前就有,只是我不知道有这么一个概念),残留烃包括三个部分:有机质的吸附量,无机矿物(岩石骨架)吸附量和孔隙中的滞留烃量。利用松辽盆地的数据分析,S1和TOC相关性极好,随着有机质丰度的增加S1增加,因此他们认为残留烃主要反映的是有机质的吸附量。这个结论让我有些诧异,用扬子地区的某井数据分析了一下,没得到类似的结果...实际上,过去是读过类似文章的,比如曾花森,霍秋立,张晓畅,鄢仁勤,姜淑杰. 应用岩石热解数据S2-TOC相关图进行烃源岩评价[J]. 地球化学,2010,06:574-579.,以及烃源岩中有效有机质的类型评价及意义,文中明确叙述为S2与TOC的散点图上,截距代表死碳,在气区是否也可以这样用?其他的一直没读太明白,暂时放下,等什么时候出现奇迹脑洞大开...

Mineralogy

第一个是关于TOC与Si或Ca含量变化的关系,分为carbonate 和silicate dominated sedimentary systems两种情况,前者TOC与silicate content正相关,后者与carbonate正相关,分析的原因如下: organic matter tends to show a good positive correlation with silicate content in carbonate dominated sedimentary systems, because increased silicate content is usually accompanied by increased nutrient supply (Littke et al., 1991; Rippen et al., 2013; Sachse et al., 2012a). By contrast, in silicate dominated systems in which nutrient supply is always sufficient, carbonate can be positively correlated to organic matter content (Sachse et al., 2012b),reflecting stronger input of aqueous(planktonic) biomass, possibly due to betterpreservation and/or dilution of organic matter rich marls by additional clay/silicate.

第二个是关于粘土矿物组合特征的分析。该区域的岩心样品经过XRD分析以后,得到的粘土矿物组分有一个明显的特点:In the upper section I/S + illite + moderate kaolinite and small amounts of chlorite are associated, and I/S + illite are associated in the lower section. I/S mixed-layer minerals are present in all the samples and slightly increase in amount with increasing depth. 教授分析其原因是蒙脱石随着埋藏温度增加,大量的转换为伊蒙混层,所以全部井段缺少蒙脱石,而伊蒙混层占主导地位,同时,随着埋深增加,伊利石逐渐增多,高岭石和绿泥石逐渐减少直至消失:With increasing burial depth, the illite content of the clays gradually increases, kaolinite and chlorite decrease, and especially in the lower section (N3040 m), kaolinite and chlorite nearly disappear.这种变化很明显,导致难以解释,教授文中推测可能是气候原因导致的,但目前没有确切的依据。

Pores and fractures

shale gas研究领域中,很多专家用很多高档仪器分析过岩石孔隙,电镜扫描基本是标配,有的专家甚至用上了岩石CT扫描技术等。当然这篇文章中,我收获最大的是教授给研究区域页岩中观察到的孔隙进行分类。In this study, interparticle pores between detrital or authigenic particles have been commonly observed, and they can be divided into three types: pores between mineral aggregates, such as calcite or quartz, pores between clay aggregates , and grain-rim interparticle pores. 从后文看,前面的三种是粒间孔(不知道有没有说对...),后面提到了粒内孔:Intraparticle pores are also present in lacustrine shales, mainly in intercrystalline dolomite , and pyrite framboids incompletely filled with clay minerals and organic matter. 教授还特意指出,很多文献里提到过有机孔,在这个区域的样品分析中并没有见到,并认为有机孔的控制因素其实还不明确,有可能与成熟度、有机质类型与丰度乃至the grain protection effect有关。请教了王教授,“the grain protection effect”应该理解为“矿物颗粒的支撑保护作用”。

裂缝的重要性不言而喻,不管是常规还是非常规,教授将这个区域的裂缝分为4类:In our study, four types of fractures in the Es3L shale are identified: tectonic fractures, abnormal pressure fractures, clay shrinkage fractures and inter-laminar fractures.

  • Tectonic fractures which have been formed under direct regional orlocal tectonic stress provide one of the most important reservoirs in the studied area.虽然有的已被充填,但整体来说是有利于压裂的。

  • Abnormal pressure fractures are mainly formed when the fluid pressure in the strata acting on the shale exceeds the fracture strength of the rocks.这种缝的主要特征是缝面irregular and its length is short, 并且很有可能被沥青充填,表明主要是由于有机质热演化过程中的异常压力形成的裂缝。

  • Clay shrinkage fractures can be caused by the transformation of clay minerals during diagenesis or low-grade metamorphism.粘土矿物的转换以及低变质作用形成的。也有可能是岩心样品表面脱水形成的,不是地层的真实反应,需要谨慎对待。

  • Inter-laminar fractures 主要出现在calcite laminae and muddy laminae表面, 形成机理也有差异,但this type of fracture is found developed in mudstones which are rich in calcite and lamellae. 这种缝 continuous and long, with width reaching several micrometers,不管怎么样,都能成为油气通道。

Shale oil resource potential

由于shale oil和shale gas的评价差异大,不能直接照搬。

  • OSI(Oil Saturation index)评价方法,即S1*100/TOC,据说只在shale oil中使用过,shale gas目前还无解。

  • 矿物脆性指数brittleness=(Quartz+Carbonate)/Total mineral。

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