JOURNAL OF STRATIGRAPHY AND SEDIMENTOLOGY RESEARCHES
Year:2011 | Volume:27 | Issue:1 (42)|Papers Count:11

Reservoir quality of Upper Dalan and Kangan carbonate reservoirs in the South Pars field has been affected by various diagenetic processes. The formation of anhydrite cement is one of the most important of these processes, developed in layered, poikilotopic, pore-filling and pervasive, nodular and sparse crystals, fracture filling and veinlet forms. These textures during different diagenetic stages, by replacement, displacement and pore occluding and in different degrees, have been affected various facies in the reservoir. The results of this study indicate anhydrite cement has mostly affected the dolomite facis than the lime facies. In addition, this also shows the effect of sulfate- rich brines in dolomitization. In the view of reservoir quality, Pore-filling and pervasive anhydrite texture is more effective than the others. As, occluding of whole pore spaces in dolograinstone and grain-dominated dolopackstone facies by this anhydrite and during shallow burial diagenesis, has resulted in extreme damaging of the reservoir quality. In contrast, the reservoir quality of these facies has been improved due to dissolution of this cement and creation of the secondary vuggy pore spaces in subsequent diagenetic stages. It can be concluded that the development and effect of the anhydrite in different textures in the reservoir, controlled by three main factors including the presence of sulfate-rich brines, initial sedimentary texture and the effect of the later diagnetic processes (i.e.dissolution and fracture).

In order to interpret the facies, depositional environment and sequence stratigraphy of the Sarvak Formation (L. Albian-Cenomanian in study area) in SW Brojen area, one stratigraphic section were studied. Eight different microfacies have been recognized, which can be grouped into four depositional environments: tidal flat, lagoon, shoal and slope. The Sarvak Formation represents sedimentation on a carbonate ramp. Three third-order sequences are identified in the Sarvak Formation, on the basis of deepening and shallowing patterns in the microfacies. The upper part of the Kazhdumi Formation is considered as the TST of sequence 1. The boundary between Kazhdumi and Sarvak formations is considered as mfs. The HST of sequence 1 is recorded in the Sarvak Formation. The SB between sequence 3 and overlying formation (Gurpi Formation) is considered as SB1.

In this study thermal modeling of Pabdeh, Gurpi, Lafan and Kazhdumi Formations in Binak oilfield is carried out using data obtained from experimental analyses including, Rock-Eval pyrolysis, Vitrinite reflectance and Pars Basin Modeler, PBM, software. Tmax and Easy RO% models both demonstrate that Gurpi, Lafan and Kazhdumi Formations are in the early and Gadvan Formation is in the late oil generation stages. Thickness of Aghajari Formation is variable in Binak oilfield. Increase in Aghajari thickness particularly in the flanks caused the underlying formations to experience higher maturity levels. For instance, Pabdeh Formation has reached oil generation stage in well number 6 due to extra thickness of Aghajari Formation. The results of thermal modeling indicate an increase in heat flow up to 80 mw/m2 in the center of the oilfield which could be due to Kharg-Mish fault passing through the center of this oilfield. Oil generation history reveals that production of oil began after the deposition of related cap rocks. Also, oil generation from Gadvan Formation was taken place before; Kazhdumi Formation during; and Pabdeh, Gurpi and Lafan Formations just after trap formation. Finally, this model suggests, oil accumulation in Asmari, Bangestan and Khami reservoirs, along with deposition of Aghajari Formation during orogeny and later erosion of 200 m of its sediments in the studied area.

The mid Cretaceous (Albia-Truonian) Sarvak Formation in the Zagros basin of SW Iran is principally composed of carbonates with minor shales and marls. This is the second major oil carbonate reservoir rock in the Zagros area. For investigation of effects of diagenetic processes and trace elements changes on the reservoir properties of this formation are studied 391 thin sections and are analyzed 40 powder samples.The early marine diagenesis is mainly represented by micritization, bioturbation, radiaxial fibrous and syntaxial overgrowth cements, and developed interparticle porosity formations. Meteoric diagenesis includes clean syntaxial and drusy cements, moldic and vuggy porosities formations. Late diagenetic processes (in shallow and deep burial stages) involve dolomitization, stylolitization, poikilotopic cement, fractures and a little vuggy and moldic porosities formation.The petrographic and elements (Sr, Mn, Na) studies indicate that aragonite was original carbonate mineralogy. Geochemical studies illustrate that these carbonates were affected by meteoric diagenesis in a closed system and trace elements changes prove that with using Sr and Mn values variations can separate facies from each other.

Pabdeh Formation in studied area (northwest of Zagros Basin and north Dezful Embayment) comprises alternation of thin to Thic limestones (lime mudstone, wackestone, packstone and grainstone) and dark to light gray shale beds, rich in planktonic microfauna. Vertical stacking of these facies suggests that depositional environment of the type section evolved gradually from an outer ramp with planktonic facies and anoxic to suboxic conditions in to a mid ramp with high energy oolitic shoal facies and inner ramp with lagoonal facies. In the mid ramp, strong storms were important for sediment transport and sedimentation. Storm deposits (carbonate tempestite facies) with their unique sedimentary structures such as erosional basal contact, hummocky cross-stratification, normal grading, ripply cross-lamination, ripple mark on top of each event bed, couplets of fine (lime mudstone) and coarse (shell-lags) laminae, deposited in this suite. Based on the same studies in the northwest of Zagros Basin, suggest a shallowing-upward trend of the facies deposited in a distally steepend carbonate ramp, wherein the depositional setting evolved from an outer ramp with pelagic and turbidite facies (Microfacies 4B and 4C) and reducing condition to oxygen depleted waters (TOC>2.5%) that gradually changed into middle ramp where marine currents (e.g. turbidity currents) had important roles in sediment reworking and redeposition.

The Cretaceous Sarvak Formation (Albian-Turonian) in the Azadegan oil Field in SW Iran is composed of a thick succession of carbonate deposits. Integrated macro and microscopic studies and seismic attributes show that reservoir interval of the Upper Sarvak Formation in the studied field consists of four depositional facies. These are: (1) Oligosteginid wackestone to packstone, (2) Bioclastic-echinoid packstone to wackestone, (3) Bioclastic-rudist boundstone and (4) Channel-filled carbonate deposits. These facies were deposited in distal open marine, proximal open marine, carbonate buildups/barriers and channel depositional setting, respectively. The comparison of these facies with the same worldwide carbonate successions show that the Upper Sarvak carbonates deposited on a carbonate ramp. Emergence of some parts of Sarvak carbonate platform at the end of Cenomanian stage resulted in development of channel deposition in the studied area.Reservoir quality of the Upper Sarvak Formation has good relationship with depositional facies, so that diagenetic controls on porosity and permeability are also facies-controlled. Mud-supported facies belong to distal open marine setting (facies 1) have low porosity and permeability. This is due to fine-grained texture and also lack of diagenetic effects. On the other hand, grain-supported facies belong to proximal open marine and carbonate build up/barrier setting (Facies 2 and 3) have relatively good reservoir quality. This is due to presence of primary interparticle pore spaces and also development of dissolution.This study highlights the facies-controlled diagenesis and its importance in prediction of the distribution of porosity and permeability ahead of exploration in hydrocarbon reservoirs as well as enhanced recovery. Therefore such study should apply to other oil and gas fields and sedimentary basins.

Permian-Triassic (P-T) hydrocarbon reservoir of the Upper Dalan and Kangan formations (equivalents of the Khuff Formation) are the main hosts of natural gas in the Persian Gulf. The purpose of this study is to investigate and compare of facies changes of Upper Dalan in the South Pars and Salman fields. To better constrain the spatial and stratigraphic distribution of the depositional facies two wells are selected. Facies analysis in the studied wells indicates that these sediments consist of limestone, dolomite and evaporite that were deposited in the inner to beginning of the middle parts of homoclinal carbonate ramp. Petrographic study led to identification fifteen facies and four distinct setting: 1-supratidal (sabkha), 2-lagoon, 3-shoal barrier and 4-open marine. This study showed that facies in Salman field were deposited in a shallower setting than the South Pars. Paleogeographic reconstruction of Arabian plate indicate that the Salman and it's adjacent fields are the result of salt tectonic. The predominance of shallower facies in the latter field could be related to this theory.

Alluvial deposits of Zayandeh Rud river bank, between Ghortan and Varzaneh area, consist of 3-4 m fining upward of gravelly, sandy and muddy sediments.They include 10 lithofacies of matrix supported gravel)Gmm), clast supported gravel (Gcm, Gci), horizontally stratification gravel (Gh), planar cross bedded gravel(Gp), trough cross bedded gravel (Gt), trough cross bedded sand (St), planar cross bedded sand (Sp), massive mud (Fm), intermittent sand and mud (Fl). All lithofacies have formed in 8 architectural elements of channel (CH), gravity flow deposits (SG), levee (LV), crevasse splay (abandoned channel) (CS, FF (CH)), sandy bed forms (SB), laminated sand sheet (LS) and lateral accretion (LA). The facies and architecture elements are mostly developed in a meandering river system in arid to semiarid climate. Deposition of debris flow sediments in lower part of succession suggest both sedimentation activity of alluvial and fluvial systems in the study area.