--> Abstract: Recognition of Palaeoexposure Surfaces within Cenomanian-Turonian Strata of Southwesterrn Iran: Implications for Reservoir Characteristics, by Elham Hajikazemi, Ihsan S. Al-Aasm, and Mario Coniglio; #90105 (2010)

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Recognition of Palaeoexposure Surfaces within Cenomanian-Turonian Strata of Southwesterrn Iran: Implications for Reservoir Characteristics

Elham Hajikazemi1; Ihsan S. Al-Aasm1; Mario Coniglio2

(1) Earth & Environmental Sciences, University of Windsor, Windsor, ON, Canada.

(2) Earth & Environmental Sciences, University of Waterloo, Waterloo, ON, Canada.

Abstract

Stable carbon and oxygen isotopes and 87Sr/86Sr ratios determined in surface and subsurface carbonates of the Sarvak Formation reveal the presence of multiple subaerial exposure surfaces that resulted from sea-level fluctuations. The sequence boundaries exhibit different degrees of geochemical alteration with more extensive alteration representing longer duration of subaerial exposure. The δ13C (range from -6.4‰ to 4.1‰, VPDB) and δ18O values ( ranges from -9.4‰ to -0.9‰, VPDB) determined for Sarvak matrix carbonates fall well within the Mid-Cretaceous marine values while the palaeoexposure surfaces are characterized by more negative δ13C and δ18O values and higher 87Sr/86Sr ratios. The most depleted δ13C values in carbonate soils formed due to subaerial exposure resulted from interaction of marine carbonates with aggressive meteoric water charged with atmospheric CO2. This caused pronounced karstification and development of favorable reservoir characteristics including effective porosity and permeability.

Introduction

Identification of subaerial exposure surfaces in carbonate rocks is essential for establishing a proper stratigraphic and diagenetic framework. Their recognition, however, is commonly challenging. The Cenomanian-Turonian Sarvak carbonates in southern Iran are no exception. This formation is regarded as a continuous succession with a major break at the upper part (e.g., Setudehnia, 1978). However in many areas a substantial amount of erosion seems to have taken place during Turonian time, removing the upper part of the succession including the older palaeoexposure surfaces (Ghazban, 2007).
Surface and subsurface sections from south western Iran (see Fig.1 for study area) were examined and sampled in order to define the relative age of the unconformities and the possibility of other as yet unrecognized depositional breaks. This was carried out using detailed petrography, stable carbon and oxygen isotope analysis in conjunction with strontium isotope records.

Regional Geological Setting

The Cenomanian-Turonian Sarvak succession in southwestern Iran is dominated by widespread shallow marine carbonates with an extensive development of rudist- benthic foraminifera bioherms/ biostorms. Sea level fluctuations, local tectonics and salt diapirism have left recognizable imprints on this succession including thickness and facies variations.

Local uplift at the end of Cenomanian caused erosion of Sarvak Formation. In the early to mid-Turonian, regional uplift and sea-level fall caused a major unconformity marked by rubbly breccia, karstification and hematite nodules. These two major Middle Cretaceous unconformities greatly influenced the development of porosity and determined the reservoir characteristics in these strata.

Material and Methods

The formation was thoroughly investigated on outcrop scale and also cores obtained from the drilled-wells. Two hundred and two samples were collected from the Sarvak Formation at its type section locality in Bangestan Mountain and well-exposed outcrops showing extensive erosional surfaces of the Upper Sarvak along the Shahneshin Mountain in NE Shiraz. A total of one hundred and fifty core samples were collected from three wells drilled through the Upper Sarvak Formation in Rag-e Sefid and Bibi Hakimeh oilfields. The samples were made into petrographic thin sections, which were stained with Alizarin red-S and potassium ferricyanide for carbonate mineral identification and studied by transmitted light. Cathodoluminescence (CL) microscopy of one hundred and thirty representative carbonate samples was carried out using a Technosyn 8200 MKII model cold cathodoluminescence stage on the unstained halves of the uncovered thin sections.

One hundred and thirty samples of the least altered carbonate matrix (i.e., micrite) and rudist shells were micro-sampled and analyzed for oxygen and carbon isotopic ratios using a Finnigan Mat Delta Plus mass spectrometer. All analyses for oxygen and carbon isotopes are reported in per mil (‰) notation relative to the Vienna Pee Dee Belemnite (VPDB) international standard. Precision for both isotopes was better than 0.05‰.

Fifty one samples from carbonate matrix and rudist shell were analyzed for Strontium isotopes on a Finnigan MAT 262 with 5 fixed collectors. NBS-87 and ocean water were used as standard references, and 87Sr/86Sr ratios were normalized to 88Sr/86Sr = 8.375209. Precision was better than 0.00010.

RESULTS

Petrography: Based on field and detailed petrographic investigations four main depositional environments were identified in the Sarvak Formation comprising the inner- ramp, mid-ramp, outer-ramp and open marine or basinal environment. Each environment is characterized by several microfacies, including rudist rudstone and rudist-benthic foraminifera grainstone which form the most porous intervals. The observed porosity types included interaparticle porosity confined to the body cavity of the rudists and also fracture, vuggy and moldic porosity in the Upper Sarvak Formation.

Petrographic and SEM observation also revealed that the internal microstructures of most of the rudist shells have high degree of preservation. Thus, the geochemical signatures obtained could well represent the most pristine values.

Stable Carbon and Oxygen: The δ13C values of the carbonate matrix range from -6.4‰ to 4.1‰ (VPDB) and δ18O values ranges from -9.4‰ to -0.9‰ (VPDB), respectively (Fig.2). The δ13C and δ18O values of the rudist shells range from 1.0‰ to 2.1‰ and -5.3‰ to -3.0‰, respectively. Most δ13C and δ18O values of the Sarvak carbonate matrix fall well within the range of the Middle Cretaceous marine carbonates (e.g., Veizer et al., 1999). The most depleted δ13C values belong to carbonate matrix of the upper most part of the Sarvak Formation in studied wells. The carbonate soil developed at the erosional surface observed and sampled in Shahneshin Mountain show δ13C and δ18O values ranging from -1.6‰ to -9.0 ‰ and -3.7‰ to -5.2 ‰, respectively.

Strontium isotopes: The 87Sr/86Sr ratios obtained in this investigation range from 0.70728 to 0.70878 for matrix and from 0.70736 to 0.70749 for rudist shells. The 87Sr/86Sr ratios obtained from argillaceous pelagic limestones in the lower Sarvak are remarkably constant, ranging from 0.707403 to 0.707451 (Fig.3). Two distinct peaks are identified along the vertical profile showing sharp increase in radiogenic 87Sr, (i.e.,87Sr/86Sr = 0.70784) at nearly 450 meters from the top of the formation, with the second peak (87Sr/86Sr = 0.70878) at the unconformity level. Stratigraphicaly the lower peak corresponds to the Lower Cenomanian sedimentary break while the higher peak represens the Turonian unconformity on top of the formation. The 87Sr/86Sr obtained in between the two peaks show ratios relatively close to marine carbonates before reaching a decrease in 87Sr/86Sr ratio of 0.70728. Highly radiogenic 87Sr/86Sr ratios have also been determined from the top parts of the subsurface sections including the core samples from Rag-e Sefid-A (87Sr/86Sr = 0.70832) and Bibi Hakimeh (87Sr/86Sr = 0.70786)

DISCUSSION

Superimposing our data on the Cenomanian-Turonian δ13C sea water curve (Voigt 2000, Jarvis et al., 2006) shows a very good correlation. Such good agreement including the similar δ13C excursions indicate that the isotope record reflects a primary marine signal. The positive δ13C value corresponds to δ13C excursion for the Mid-Cenomanian Event (MCE) as indicated by Jarvis et al. (2006). The δ13C excursion can be correlated both in surface and subsurface sections.

It is important to note that the well-documented Cenomanian/Turonian boundary Event is missing in the studied Upper Sarvak sections compared to the global curve. This is interpreted as being the results of extensive erosion of the Upper Sarvak carbonates by the regional Turonian or local Cenomanian/Turonian boundary unconformities.

On a vertical profile, the Sarvak limestones also show negative δ13C and δ18O values (see Fig.1). The prominent negative shifts in δ13C coupled with δ18O values are interpreted to be the result of diagenetic alteration at or below the palaeoexposure surfaces. This could have occurred by repeated subaerial exposures of the carbonate platform at various time and durations. Multiple sea-level falls exposed the Sarvak Formation and subjected the carbonates to chemically aggressive meteoric fluids, causing the corresponding depletion in 13C and 18O. This is interpreted to be the expression of hydrologically active locations in which water-rock interaction was maximized.

The sharp increase in 87Sr/86Sr ratio of carbonate matrix measured at the uppermost part of all the surface sections and wells shows a departure from normal sea water curve (e.g. McArthur, 2001) implying alteration associated with fluids other than normal sea water. These ratios are by far more radiogenic than any other values along the curve. Thus, it is assumed that highly radiogenic 87Sr/86Sr values obtained from surface and subsurface sections are associated with regression and palaeoexposure surfaces while majority of the 87Sr/86Sr ratios vary little over major part of the sections falling well within the Cenomanian-Turonian marine ratios.

Based on the petrographic and well-log information the top of the formation has been identified in the subsurface sections. In most of the studied sections, the formation top could be recognized by depleted δ13C peaks which are also corresponding to highly radiogenic 87Sr/86Sr below this horizon.

Conclusion:
The results obtained in this investigation indicate repeated sea-level falls of different durations corresponding to palaeoexposure surfaces within the Sarvak Formation in both surface and subsurface sections. The negative δ13C and δ18O values associated with highly radiogenic 87Sr/86Sr ratios displayed in the isotope profiles are reliable indicators of subaerially exposed surfaces in the Sarvak carbonates and modification of isotope values due to meteoric diagenetic effects.
Determination of these geochemical values from subsurface related to the presence of subaerial exposure is of vital importance because these surfaces greatly influenced the development of porosity and associated reservoir characteristics in various areas.

Good agreement between isotope signatures in different surface and subsurface profiles confirm the synchronicity of changes and diagenetic events throughout the Cenomanian-Turonian, illustrating the potential of the composite isotope reference curve as a primary criterion for regional correlation.

References

Hajikazemi, E., Al-Aasm, I.S., Coniglio, M., in press, Subaerial Exposure and Meteoric Diagenesis of the Cenomanian-Turonian Upper Sarvak Formation, southwestern Iran, Geological Society of London.

Jarvis, I., Gale, A.S., Jenkyns, H.C., Pearce, M.A., 2006. Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian- Campanian (99.6-70.6 Ma). Geological Magazine 143, 561-608.

McArthur, J.M., Howarth, R.J., Bailey, T.R., 2001. Strontium isotope stratigraphy: LOWESS Version 3. Best-fit to the marine Sr-isotope curve for 0 to 509 Ma and accompanying look-up table for deriving numerical age. J. Geol. 109, 155-170.

Setudehnia, A. 1978. The Mesozoic succession in S.W. Iran and adjacent areas. Journal of Petroleum Geology, 1, 3-42.

Veizer, J., Ala, D., Azmy, K., Brukschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawelleck, F., Podlaha, O., Strauss, H., 1999. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chem. Geol. 161, 59- 88.

Voigt, S., 2000. Cenomanian-Turonian composite δ13C curve for Western and Central Europe: the role of organic and inorganic carbon fluxes. Palaeogeography, Palaeoclimatology, Palaeoecology. V. 160, 91-104.

681051_A.jpgFigure 1 Map of southwest Iran showing the location of the study area. Inset map at bottom left.

681051_B.jpgFigure 2.Carbon and Oxygen isotope profile of the Sarvak Formation at the type section locality, Bangestan Mountain

681051_C.jpgFigure 3. 87Sr/86Sr profile of the Sarvak Formation along the type section, Bangestan Mountain