Department of Earth Sciences, Royal Holloway University of London, Egham, TW20 0EX, UK.
Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18002, Granada, Spain.
Sci Rep. 2021 Jan 21;11(1):1975. doi: 10.1038/s41598-021-81687-6.
Bioturbation is an important factor for reservoir quality due to the modification of host rock petrophysical properties (i.e., porosity, permeability, and connectivity). However, there is no predictable relationship between bioturbation and its effect on rock properties, due to the variability of the involved ichnological features. A detailed ichnological analysis is necessary to determine how bioturbation affects petrophysical properties in a bioturbated reservoir. Traditionally, ichnological features such as density, tiering, size, orientation, architecture, and fill, have been considered. However, other properties have been undervalued as is the case of lining. Here, we present a detailed study on the effects of Macaronichnus burrows, an ichnotaxon usually related to hydrocarbon exploration due to its high concentration in rock notably affecting petrophysical properties. Macaronichnus, a subhorizontal cylindrical burrow, is characterized by a well-defined and developed outer rim surrounding the tube core. Our data indicates a clear zonation in porosity according to burrow structure, with the lowest porosity in the tube core and higher values associated with the surrounded rim. This duality is determined by the tracemaker grain selective feeding activity and the consequent concentrated cementation. The organism concentrates the lighter minerals in the tube core fill during feeding, favoring post-depositional cementation during diagenesis and this results in lower porosity than the host rock. However, heavy minerals, mainly glauconite, are located in the rim, showing higher porosity. Our results support the view that ichnological analyses are essential to determine reservoir quality in bioturbated reservoirs, evidencing that other ichnological properties in addition to those traditionally considered must be evaluated.
生物扰动是影响储层质量的一个重要因素，因为它改变了母岩的岩石物理性质（即孔隙度、渗透率和连通性）。然而，由于涉及的遗迹特征的可变性，生物扰动与其对岩石性质的影响之间没有可预测的关系。需要进行详细的遗迹学分析，以确定生物扰动如何影响生物扰动储层的岩石物理性质。传统上，人们考虑了遗迹学特征，如密度、分层、大小、方向、结构和填充。然而，其他属性也被低估了，例如衬里。在这里，我们对 Macaronichnus 洞穴的影响进行了详细研究，Macaronichnus 是一种通常与烃类勘探有关的遗迹化石，因为它在岩石中的高浓度显著影响岩石物理性质。Macaronichnus 是一种水平圆柱形洞穴，其特征是围绕管芯的外边缘轮廓清晰、发育良好。我们的数据表明，根据洞穴结构，孔隙度有明显的分带，管芯中的孔隙度最低，而与外边缘相关的孔隙度较高。这种二元性是由轨迹制造者的颗粒选择性摄食活动和随后的集中胶结决定的。在摄食过程中，生物体将较轻的矿物集中在管芯填充物中，在成岩作用过程中有利于后期胶结，导致孔隙度低于母岩。然而，重矿物，主要是海绿石，位于边缘，显示出较高的孔隙度。我们的结果支持这样一种观点，即遗迹学分析对于确定生物扰动储层的储层质量至关重要，这表明除了传统上考虑的那些特征外，还必须评估其他遗迹学特征。
