A Comparison of Various Diagenetic Controls by Zeolites on Reservoir Quality: Permian–Triassic Conglomerates, Junggar Basin, Northwestern China

Evaluating the sedimentological and diagenetic impacts on terrestrial lacustrine fan delta sandy conglomerates reservoir quality: Insights from the Triassic Baikouquan Formation in the Mahu sag, Junggar Basin, Western China

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

کربنات‌های سازند جهرم به سن ائوسن و سازند آسماری به سن الیگوسن- میوسن، مخازن میدان خشت را در ناحیة فارس واقع در حوضة فورلندی زاگرس تشکیل می‌دهند. در این پژوهش ویژگی‌های مخزنی بخش بالایی سازند جهرم در میدان خشت براساس تلفیق نتایج آنالیز رخساره‌ای و ویژگی‌های دیاژنزی نمونه‌ها در چهارچوب تخلخل و تراوایی بررسی شده است. سازند جهرم در پهنة فارس با لیتولوژی غالب آهکی در یک رمپ کربناته با تغییرات زیاد در ویژگی‌ها و کیفیت مخزنی نهشته شده است. بررسی‌های پتروگرافی به شناسایی پنج ریزرخسارة کربناته منجر شد. پنج گروه سنگی در چاه خشت-2 با در نظر گرفتن کنترل‌کننده‌های اولیه و ثانویه در توزیع نوع و اندازة منافذ شناسایی شد. از گونة سنگی 1 به سمت گونة سنگی 5، کیفیت مخزنی افزایش می‌یابد. دیاژنز به دو صورت افزاینده و کاهندة تخلخل و تراوایی بر کیفیت مخزنی تأثیر گذاشته است. کراس پلات تخلخل و تراوایی همراه با بررسی‌های پتروگرافی مقاطع نازک نشان می‌دهد توسعة سیمان انیدریتی به‌صورت فراگیر و تراکم، بیشترین تأثیر را بر کاهش کیفیت مخزنی داشته‌اند؛ در حالی که دولومیتی‌شدن، شکستگی و انحلال نومولیتس‌ها نقش مهمی در افزایش کیفیت مخزنی ایفا کرده‌اند؛ بنابراین ویژگی‌های کلی مخزن جهرم در میدان خشت، عمدتا با ویژگی‌های دیاژنتیکی شکل گرفته است. استفاده از نرم‌افزار سیکلولاگ در چاه خشت-2 و چاه کمکی خشت-3 به شناسایی دو چرخة رسوبی برای سازند جهرم منجر شد. روند منفی منحنی تغییر طیفی (پایین‌آمدن سطح آب دریا) در چرخة رسوبی دوم دربرگیرندة بخش بالایی سازند جهرم (توالی مطالعه‌شده) است که کیفیت مخزنی متوسط تا بالایی دارد.

The sandstones of the Proterozoic Rajgarh Formation are texturally immature and compositionally sub-mature quartz arenite to arkose in composition. The reservoir quality of sandstones is strongly influenced by depositional facies and various types of diagenetic modifications. A significant reduction in primary porosity in the studied sandstone is mainly due to high mechanical compaction followed by chemical compaction rather than the cementation process. Arkose from the river-dominated estuary facies show infiltrated clay coating, feldspar dissolution, iron-oxide cementation, leaching of detrital by iron-oxide cement, kaolinization of feldspar, the fracturing of detrital, partial albitization and folding of mica. The pore-lining clay has helped in retaining primary porosity, and leaching and dissolution processes were the main process generating secondary porosity in this facies. Tidal flat sub-arkose petrofacies display compaction of micas and unstable rock fragments into the pseudomatrix, expansion of muscovite by kaolinite, and pore-filling silty matrix that occupied intergranular spaces. Quartz-arenite petrofacies from shallow marine deposit show grain replacive kaolinite, pyrite precipitation, glauconite authigenesis, muscovite bending, albitization of plagioclase, feldspar overgrowths, dissolution, and grain fracturing. Pyrite plays a significant role in the reduction of both primary and secondary porosity by precipitating in intergranular spaces and in dissolution voids of feldspar. Moreover, fractures in detrital act as a second cycle porosity generation process in the shallow marine petrofacies. The linking between the diagenetic processes and depositional facies provides a geologic model for hydrocarbon exploration through reservoir quality prediction. Therefore, the petrographic observations indicate quartz-arenite (shallow marine facies) and sub-arkosic sandstones (tidal flat facies) are the least suitable latent reservoir because of their lowest primary and secondary porosity and unconnected pore spaces. Arkosic (river-dominated estuary facies) compositions are considered to be appropriate frameworks for potential reservoir rocks. This study reveals that the depositional environment largely controls the diagenetic pathways and hence, influences the reservoir quality.

Reservoir quality variations within a conglomeratic fan-delta system in the Mahu sag, northwestern Junggar Basin: Characteristics and controlling factors

Role of depositional and diagenetic controls on reservoir quality of complex heterogenous tidal sandstone reservoirs: An example from the Lower Goru formation, Middle Indus Basin, Southwest Pakistan

Palaeo‐geomorphology can exert significant effects on reservoir quality via controlling the distribution of depositional facies and diagenetic alterations. However, little attention has been paid to the studies of the reservoir variations and heterogeneities under distinctive valley‐monadnock palaeo‐geomorphology. Taking the Lower Cretaceous Qingshuihe Formation (K1q) in the Junggar Basin as an example, this study improves the current understanding of reservoir potential within topography of this kind by examining the link between depositional facies, diagenesis and their impact on reservoir quality. In total, nine types of lithofacies can be identified in the K1q reservoirs, which can be further grouped into lowstand systems tract (LST) gravelly braided river and debris flow deposits and transgressive systems tract (TST) sandy braided river delta deposits. These reservoirs are rich in rock fragments and characterized by moderate porosity and moderate to low permeability with development of intergranular and dissolution pores. Generally, detrital composition has a greater impact on reservoir quality of the LST conglomerates than on that of the TST sandstones. However, texture maturity only shows positive correlation with physical properties of the sandstone reservoirs. The mechanical compaction was responsible for the major destruction of the initial porosity. However, the large heterogeneities in calcite cements account for the extremely heterogeneous reservoir quality. Calcite cements formed during eodiagenesis usually show poikilotopic fabric and preferentially concentrated inthe basal coarser units/facies within inidividual fining‐upward successions. Consequently, the upper finer units/facies became more favourable for the secondary dissolution and oil charging processes during mesodiagenesis. In contrast, the amounts of authigenic quartz and clay minerals are relatively low. Nevertheless, the contents of different authigenic clay minerals show a variety of relationships with the reservoir quality. The valley‐monadnock topography spatially controls the scales of the LST and TST reservoirs. The simultaneously evolutionary processes between the back‐filling of the valleys and weathering of the monadnocks resulted in the prolonged residence time and large area of depositional water contacting with the weathering crust. This may give rise to the formation of alkaline pore fluids and facilitate the nucleation and growth of eogenetic carbonate cements and grain‐coating chlorite. Furthermore, the reservoir quality generally displays an increasing trend from the monadnock to the valley thalweg due to the increase of hydrodynamic strength and the less influence of mélange debris flow deposits.

Mineral dissolution and precipitation is an important factor affecting pore genesis and hydrocarbon accumulation in sedimentary basins. Differential diagenetic processes at different hydrocarbon charging periods generally cause disparate effects on reservoir quality, which is important for reservoir evaluation and prediction. Focusing on this question, petrologic work, quantitative analysis on reservoir quality including porosity, permeability, and micro-scale X-ray computer tomography were conducted in conglomeratic reservoirs of the late Permian Upper Wuerhe Formation (P3 w) in the Shawan Sag, Junggar Basin, northwestern China. The results show that tuff and volcanic debris are enriched in the formation. Laumontites generally occur as cements in the reservoirs due to the alteration of volcanic materials and small amounts of early-stage calcite precipitated during eodiagenesis. Partial laumontite and calcite cements, as well as some debris and feldspar, are differentially dissolved due to differential hydrocarbon charging. During the continuous compaction of the reservoir, first-stage hydrocarbon charging occurred in the middle Jurassic when primary porosity almost disappeared. Hydrocarbon charging inhibited the precipitation of laumontite and calcite, promoting their dissolution. Second-stage hydrocarbon charging in the early Cretaceous further caused more extensive dissolution of laumontite and calcite, forming more than 70% secondary porosity. Authigenic quartz, kaolinite, and late-stage calcite are precipitated as the associated minerals of laumontite dissolution. This study presents the significance of reservoir reconstruction after hydrocarbon charging and suggests prospective hydrocarbon accumulations in deeply buried clastic reservoirs when hydrocarbon supply is sufficient from source rocks.

Diagenetic controls on the reservoir quality of tight reservoirs in digitate shallow-water lacustrine delta deposits: An example from the Triassic Yanchang Formation, southwestern Ordos Basin, China

Depositional and diagenetic controls on the reservoir quality of marginal marine sandstones: An example from the Early Devonian subbat member, jauf formation, northwest Saudi Arabia

The Neogene sandstones offer hydrocarbon reservoirs in the offshore area of the Bay of Bengal (Bangladesh), but their reservoir quality is not well known. Understanding the impact of depositional and diagenetic controls on reservoir quality is crucial to hydrocarbon exploration and production in reservoir sandstones. This study aims to investigate the depositional and diagenetic controls on reservoir quality of the Neogene Surma Group from the Sangu Gas Field (the only offshore gas field), in the Bay of Bengal, Bangladesh using a combination of petrographical, petrophysical, X‐ray diffraction, scanning electron microscope fitted with energy dispersive spectra and core analysis data. Diagenetic modifications to the sandstones include mechanical and ductile grain compaction followed by clay cementation (dominantly chlorite, illite–smectite and minor kaolinite), poikilotopic calcite cement and subordinate amounts of quartz cement, as well as partial dissolution of feldspars and lithic fragments. Sandstones with primary porosities ≥20% have average intergranular volume value of 33%, although permeability in most sections of the reservoir falls below 8–10 mD. There has also been localized creation of secondary porosity due to dissolution. The average intergranular porosity lost by compaction is 14% (ranges from 3% to 37%), and the average porosity lost by cementation is 10% (ranges from 3% to 37%). This study shows that the reservoir quality is predominantly controlled by depositional environment (ductile grain supply, clay content), a diagenetic processes (compaction followed by clay cement and early to intermediate poikilotopic calcite cement) and will provide insights into the reservoir quality evaluation in the offshore areas of the Bengal Basin.

The sedimentary and diagenetic control on tight sandstone reservoir quality is a key focus of hydrocarbon exploration. This paper compares reservoir characteristics, such as diagenetic processes, pore types, and porosity evolution, of cores and outcrops of the Yanchang Formation, the Ordos Basin, northwestern China, to identify diagenetic trends and processes responsible for improving reservoir quality and to develop a model for identifying reservoir sweet spots in tight sandstone formations. Samples of the Yanchang Formation sandstone are taken from wells and adjacent outcrops in the Ordos Basin. No significant differences were found in porosity-destructive diagenesis such as compaction and cementation between the core and outcrop samples. However, the effects of dissolution are far more pronounced in cores than in the outcrop, resulting in a higher core sample porosity. High-density sampling of the outcrop revealed that the reservoir sweet-spot model is controlled by sedimentation, whereas three distinct vertical diagenetic zones were identified in adjacent wells. A comprehensive analysis of cores and outcrop samples shows that both areas experienced similar sedimentation and early diagenetic patterns. However, the diagenetic processes experienced by core and outcrop sandstones diverged during the later part of burial and uplift, with dissolution being responsible for the contrasting reservoir porosities between core and outcrop.

This study states the spatial and temporal distribution of diagenetic alterations and their influence on reservoir quality in the lower member of Shawan sandstones of the Chepaizi Uplift in the Junggar Basin, NW China. Sandstones in this unit, which contains heavy oil, are composed of mostly lithic arkose deposited in a braided fluvial environment. The diagenetic processes that have affected the sandstones include mechanical compaction, cementation by carbonates, quartz, and clay minerals, and dissolution of framework grains and cements. Relatively good reservoir quality of the studied arkosic heavy oil reservoir rocks deposited in the braided fluvial environment occurs when feldspar, lithic fragments, and calcite cement experience dissolution in channel lag sandstones. However, the pervasive distribution of early carbonate cements mainly concentrated in braid bar deposits has resulted in a great reduction of reservoir quality. This study reveals that understanding of diagenetic evolution of braided fluvial sediments is helpful for providing a better elucidation and prediction of reservoir quality, and the results of this study can be applied to guide the direction for exploration and production of heavy oil.

Diagenetic controls on the quality of the Middle Permian Lucaogou Formation tight reservoir, southeastern Junggar Basin, northwestern China

Sedimentological and diagenetic control on the reservoir quality of deep-lacustrine sedimentary gravity flow sand reservoirs of the Upper Triassic Yanchang Formation in Southern Ordos Basin, China