The Role of Clay Minerals in the Petrophysical Properties of Carbonate Rocks from the Cotinguiba Formation (Sergipe-Alagoas Basin, Brazil)

Q4 Social Sciences
Clarissa Oliveira Da Luz, Gerson Cardoso Da Silva Jr., Mariléa Gomes Dos Santos Ribeiro
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引用次数: 0

Abstract

The Cotinguiba Formation, of Cretaceous age, located in the Sergipe-Alagoas Basin, Brazilian Northeastern coast, comprises carbonate breccias and carbonate shales interlayered with fine to coarse clastic sediments (mudstones, marls), deposited in the maximum sea level episode during the Cenomanian-Coniacian transgressive event. Core sections were selected from the formation materials, from which 20 plugs were obtained, representing the main carbonate horizons present in Cotinguiba Formation. The selected samples were analyzed with petrophysical and physical methods (air porosimetry, mercury injection, and acoustic wave velocity) and mineralogical methods (optical microscopic analysis, x-ray diffraction (XRD) and scanning electron microscopy (SEM)). Results showed that the clay minerals’ content in the carbonates varies from 2% to 20%, with illites/micas and interstratified illite/smectite as the most abundant clay minerals, containing also traces of chlorite and kaolinite, and presence of palygorskite varying from trace to 8%. According to the percentages of calcite, dolomite, and siliciclastic + clay minerals obtained in the XRD analysis, the carbonates were lithologically identified as limestone, impure limestone, dolomitic limestone, impure dolomitic limestone, impure calcitic dolostone, and impure dolostone. The rocks have a good porosity, varying from 6% to 20% in the carbonates, predominantly related to microporosity. Clay minerals’ content influenced the grain density and acoustic properties, albeit not having the same role with porosity and permeability. SEM images, though, show that they have an important role in microporosity. Grain density variation related to phyllosilicate content was observed more significantly in impure limestones with a clay content higher than 8%, and a more pronounced decrease was observed in samples with palygorskite. Microporosity is the main factor for reducing Vp and Vs velocities in those carbonates, with values not exceeding 5341 m/s for Vp and 3026 m/s for Vs but, when clay content is higher than 4%, the Vp and Vs wave velocities do not exceed 4100 m/s and 2480 m/s, respectively. Therefore, the research allowed evaluating the influence of mineralogical and textural properties in the petrophysical properties of Cotinguiba Formation.
粘土矿物在巴西Sergipe-Alagoas盆地Cotinguiba组碳酸盐岩物性中的作用
Cotinguiba组位于巴西东北海岸Sergipe-Alagoas盆地的白垩纪,由碳酸盐岩角砾岩和碳酸盐岩页岩与细至粗碎屑沉积物(泥岩、泥灰岩)夹层组成,沉积于塞诺曼-科尼亚契海侵事件期间的最高海平面时期。从地层材料中选择了岩心剖面,从中获得了20个桥塞,代表了Cotinguiba组的主要碳酸盐岩层位。采用岩石物理和物理方法(空气孔隙度法、压汞法、声波速度法)和矿物学方法(光学显微镜分析、x射线衍射(XRD)和扫描电镜(SEM))对所选样品进行分析。结果表明:碳酸盐中粘土矿物含量在2% ~ 20%之间,以伊利石/云母和层间伊利石/蒙脱石含量最多,绿泥石和高岭石含量也在微量~ 8%之间;根据XRD分析得到的方解石、白云石、硅塑性+粘土矿物的百分比,岩性鉴定为灰岩、不纯灰岩、白云岩、不纯白云岩、不纯方解石白云岩、不纯白云岩。岩石孔隙度较好,碳酸盐岩孔隙度在6% ~ 20%之间,主要与微孔隙有关。粘土矿物含量对颗粒密度和声学性质有影响,但对孔隙度和渗透率的影响不同。然而,SEM图像显示它们在微孔隙度中起着重要作用。在粘土含量高于8%的不纯石灰石中,颗粒密度变化与层状硅酸盐含量的关系更为显著,而在含坡缕石的石灰石中,颗粒密度变化更为明显。微孔隙度是降低Vp和Vs波速的主要因素,Vp波速不超过5341 m/s, Vs波速不超过3026 m/s,但粘土含量大于4%时,Vp波速不超过4100 m/s, Vs波速不超过2480 m/s。因此,该研究可以评价矿物学和结构性质对Cotinguiba组岩石物理性质的影响。
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来源期刊
Anuario do Instituto de Geociencias
Anuario do Instituto de Geociencias Social Sciences-Geography, Planning and Development
CiteScore
0.70
自引率
0.00%
发文量
45
审稿时长
28 weeks
期刊介绍: The Anuário do Instituto de Geociências (Anuário IGEO) is an official publication of the Universidade Federal do Rio de Janeiro (UFRJ – CCMN) with the objective to publish original scientific papers of broad interest in the field of Geology, Paleontology, Geography and Meteorology.
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