In this paper, the importance of Lithocodium–Bacinella facies (LB facies) as reservoir rock in the Lower Cretaceous carbonates of the Persian Gulf will be addressed. The studied interval belongs to the reservoir zone of upper Fahliyan Formation, which occurs in one of the giant offshore oilfields. A detailed examination of a continuous core (58 m) from this formation allows clarifying the relationship between facies distribution, diagenetic evolution, and porosity development in function of relative sea level changes. Seven facies types are identified based on core descriptions and microfacies analysis. These facies are interpreted to have been deposited in lagoonal, shoal, and open marine settings during Valanginian–Hauterivian times. Petrographic studies and stable carbon and oxygen isotope analyses indicate that these facies have been affected by different diagenetic processes, starting with early marine diagenesis and followed by meteoric and/or burial diagenesis. Based on facies stacking patterns and evidence of subaerial exposures, four high-frequency sequences are interpreted in terms of variations in accommodation space. Combination of facies analysis, diagenetic studies and poroperm data reveals (with the exception of sequence I) that the lower transgressive and the upper highstand intervals of these sequences show different facies types, diagenetic modifications and reservoir properties. The transgressive system tracts (TST’s) correspond to tight zones (Z-1, 3, 5), with poor reservoir quality (Φ < 15% and K < 1 md). These intervals are composed of an alternation of open to restricted marine facies. The facies are characterized by extensive bioturbation, high clay content (argillaceous limestones), displaying marine (micritization) and shallow to deep burial diagenetic processes (pyrite mineralization and solution seams). The reservoir intervals preferentially occur within the highstand system tracts (HST’s), which are commonly composed of vertically stacked Lithocodium-Bacinella facies. This facies with aggradational patterns is capped by subaerial exposure surfaces at the top of the sequences and is associated with leaching and cementation. Meteoric dissolution during subaerial exposure resulted in the formation of vuggy and moldic pores as the main pore types in these system tracts, which survived in significant amounts until present-day. Therefore, the LB facies usually have greater than 15% porosity and 1 md permeability. Diagenetic studies in these units show that the marine diagenesis was interrupted by subaerial exposure and meteoric diagenesis before burial and development of stylolites. Our findings show that despite the uncertain taxonomy and paleoenvironmental setting of Lithocodium aggregatum and Bacinella irregularis, the LB facies has great reservoir importance.

中文翻译：

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波斯湾盆地东部下白垩统储层岩藻-芽孢杆菌相成岩作用及储层特征

在本文中，将讨论 Lithocodium-Bacinella 相（LB 相）作为波斯湾下白垩统碳酸盐岩储集岩的重要性。研究层段属于上法力岩组储集层，产于海上大型油田之一。对来自该地层的连续岩心 (58 m) 进行详细检查，可以阐明相分布、成岩演化和孔隙度发展之间的关系，以相对海平面变化为函数。根据岩心描述和微相分析确定了七种相类型。这些相被解释为在 Valanginian-Hauterivian 时期沉积在泻湖、浅滩和开阔的海洋环境中。岩石学研究和稳定的碳和氧同位素分析表明，这些相受到不同成岩过程的影响，从早期海洋成岩作用开始，然后是陨石和/或埋藏成岩作用。基于相堆叠模式和地下暴露的证据，四个高频序列根据住宿空间的变化进行解释。结合相分析、成岩研究和poroperm 数据揭示（除了层序I）这些层序的下海进层段和上层高水位层段显示出不同的相类型、成岩改造和储层性质。海侵系统域 (TST) 对应于致密带（Z-1、3、5），储层质量较差（Φ <15% 和 K <1 md）。这些层段由开放到受限海相的交替组成。这些相具有广泛的生物扰动、高粘土含量（泥质灰岩）、海相（微晶化）和浅至深埋藏成岩过程（黄铁矿矿化和溶解缝）的特征。储层层段优先出现在高位系统域 (HST) 内，通常由垂直堆积的 Lithocodium-Bacinella 相组成。这种具有加积模式的相被层序顶部的地下暴露表面覆盖，并与浸出和胶结有关。地下暴露过程中的陨石溶解导致这些系统域中主要孔​​隙类型的孔洞和霉菌孔的形成，这些孔洞和霉菌孔大量存在直到今天。所以，LB 相通常具有大于 15% 的孔隙度和 1 md 的渗透率。这些单元的成岩作用研究表明，海相成岩作用被地下暴露和大气成岩作用中断，然后才埋藏和发育缝合线。我们的研究结果表明，尽管聚石藻和不规则芽孢杆菌的分类学和古环境环境不确定，但 LB 相具有重要的储层重要性。