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Facies control on selective dolomitization and its impact on reservoir heterogeneities in the Samana Suk Formation (middle Jurassic), Southern Hazara Basin (NW Himalaya, Pakistan): an outcrop analogue
Geosciences Journal ( IF 1.2 ) Pub Date : 2019-11-12 , DOI: 10.1007/s12303-019-0026-7
Mumtaz M. Shah , Hamad U. Rahim , Aosaf Hassan , Muhammad R. Mustafa , Imran Ahmad

This studied rocks form part of the southern Hazara basin in the NW Himalayan Fold and Thrust belt, Pakistan. Thick inner to mid ramp carbonate platform deposits of Samana Suk Formation (Bajocian-Callovian) mostly composed of medium to thick bedded limestone with occasional dolostone beds are exposed in various localities. In the present studies, attempt has been made to understand degree of diagenetic alterations (i.e., dolomitization) within various limestone facies due to fluid-rock interaction phenomenon, and its impact on the reservoir behavior of altered rocks based on field investigations, petrographic studies, stable isotope signatures and porosity/permeability analysis. Field observations revealed various limestone types (oolitic, burrowed, fossiliferous, micritic and sandy), whereas dolomite occurs as completely replacive phase (Dol-I), and partially replacive patchy bodies (Dol-II) respectively. Petrographic studies showed various limestone and dolomite facies, which include: (i) Grainstone facies (bioclastic grainstone, peloidal grainstone, ooidal grainstone and pel-bioclastic grainstone), (ii) Packstone facies (bioclastic packstone and peloidal packstone), (iii) Wackestone facies (bioclastic wackestone), (iv) Mudstone facies (lime mudstone, and (v) Dolomite facies (coarse crystalline strata-bound Dol-I, and fine crystalline patchy Dol-II) respectively. O/C isotope analysis revealed that Dol-I shows signatures of δ18O (−5.84 to −3.91‰ V-PDB), and δ13C (+0.6 to +2.37‰ V-PDB) are within the limit of the carbonate marine seawater signatures, hence originated from sea-water or modified sea-water, whereas Dol-II exhibited depleted δ18O values (−6.88 to −5.87‰ V-PDB) and slightly depleted δ13C signatures (+0.968 to +1.85‰ V-PDB), indicating high temperature dolomitizing fluids. During early stage of marine diagenesis, which resulted in the cementation of pores within the grainstone-packstone facies, whereas mudstone-wackestone facies remained unaltered due to low porosity and permeability. Late stage dolomitization caused partial to complete alteration of mudstone-wackestone facies, in contrast grainstone-packstone facies are not affected by dolomitizing fluids due to the fact that the pore network of these coarser facies had already been occluded by the preceding marine cementation event. Porosity and permeability analyses revealed relatively high porosity values (4–8%) and permeability values (11.5 mD) in the dolomitized facies, whereas unaltered limestone facies showed considerably low porosity/permeability values (> 1%). In conclusion, less porous/permeable fine-grained facies evolved into more porous and permeable units due to the interaction of dolomitizing fluids of hydrothermal origin, which confirms that the original sedimentary texture of rock has implications on the selectivity of any diagenetic alteration.

中文翻译:

南部哈扎拉盆地(巴基斯坦西北喜马拉雅)Samana Suk组(侏罗纪中部)选择性白云石化的相控及其对储层非均质性的影响:

这项研究研究的岩石是巴基斯坦西北喜马拉雅山褶皱和冲断带中哈扎拉盆地南部的一部分。萨马纳苏克组(Bajocian-Callovian)的内部至中层斜坡碳酸盐台地沉积物主要分布在中,厚层状石灰岩和偶有白云岩层的地方。在目前的研究中,根据野外调查,岩相学研究,已经尝试了解由于流体-岩石相互作用现象而引起的各种石灰岩相中的成岩作用变化(即白云石化)程度及其对蚀变岩石储层行为的影响,稳定的同位素特征和孔隙度/渗透率分析。野外观察发现,各种石灰岩类型(橄榄岩,洞穴,化石,微粉岩和砂质),而白云岩则以完全置换相(Dol-I)出现,和部分替换的斑块体(Dol-II)。岩石学研究显示了各种石灰岩和白云岩相,其中包括:(i)颗粒岩相(生物碎屑颗粒岩,倍性颗粒岩,卵形颗粒岩和象素-生物碎屑颗粒岩),(ii)堆积石相(生物碎屑堆积石和倍性堆积岩),(iii)瓦克石(iv)泥岩相(石灰泥岩)和(v)白云岩相(粗晶层结合的Dol-I和细晶斑状Dol-II),O / C同位素分析显示Dol-我显示了δ的签名18 O(-5.84到-3.91‰V-PDB),和δ 13 C(0.6至2.37 +‰V-PDB)是碳酸酯海洋海水签名的限度内,因此来自海水或改性海基水,而DOL-II表现出耗尽δ 18个O值(-6.88到-5.87‰V-PDB)和稍微贫δ 13C信号(+0.968至+ 1.85‰V-PDB),表明高温白云石化流体。在海洋成岩作用的早期阶段,导致了胶结-砾岩相内孔隙的胶结,而由于低孔隙度和低渗透率,泥岩-瓦克石相保持不变。后期白云石化作用导致泥岩-瓦克石相的部分或全部改变,相反,由于这些较粗相的孔隙网络已经被先前的海相胶结作用闭塞了,因此颗粒-堆积岩相不受白云石化流体的影响。孔隙度和渗透率分析显示,在白云石化相中孔隙度值较高(4-8%),渗透率值(11.5 mD),而未改变的石灰岩相孔隙度/渗透率值则相对较低(> 1%)。
更新日期:2019-11-12
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