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________________________________________________________ ( ) * - - . . . ) ( . A B D C E . . F E D C B A . . . Chilostomellids . F . EDS . . Petrography - Stratigraphy Study of the Key Beds in Member 1 of the Gachsaran Formation (Cap Rock), Agha Jari Oilfield Abstract The Member 1 of the Gachsaran Formation is considered as the Cap Rock in petroleum domain of south of Iran. The cap rock has consisted of a series of Key Beds, which their identification is necessary to casing point and reducing drilling risk (with regards of pressure change from high to low in member 1). In Agha Jari oilfield, the cap rock is divided into 6 key beds which can be correlated through the field. These are A, B, C, D, E and F, respectively from top to bottom. The key bed A underlies the main salt of Gachsaran Formation (member 2) and is mainly composed of anhydrite. The key bed B with the lithology of bituminous shale is the most important ones in the cap rock. The key beds C & D are calcareous and called Chilostomellids bearing layers. The key bed E is poor in fossil content. The bottommost key bed is F and is consisted of mudstone and wackstone. These key beds are evaluated with Gamma Ray and Sonic well logs. Chemical analysis 2027 ________________________________________________________ of selected samples carried out with SEM-EDS analyzer. These data as well as petrography of key beds indicate that depositional environment is platform edge/Sabkha which periodically charged by sea water. The result is also verified by Carbon and Sulfur isotopic data of the selected samples from carbonate and anhydrite layers. - (Key Beds) . .( ) Slinger, 1948 & Child,1944; Watson, 1960) .(Stone & Watson, 1962 1949 - . . ° ° ' (SEM-EDS) . . Humble . F . E D C B A . -A . . Lath 2028 - - ________________________________________________________ . CarbE CarbD2 CarbD ( ) . . . . B (Main Salt) -B - - . .( ) . . A CarbD2 CarbD . -C (Upper Cap - - Upper Chilostomellid Limestone Rock Limestone) . . . Dunham (1962) Lamelibranch Gastropod Chilostomellid . -D - . (Lower Cap Lower Chilostomellid Limestone . . C -E . .(Slinger, 1948) Chilostomellid . 2029 Rock Limestone) - - ________________________________________________________ -F - - . . (Folk, 1962) (Polymorphinid) Chilostomellid . . (SEM-EDS) . . A F E D . ( / - ) ‰ 34S (- / ) ‰ 13C . . . (EDS) - : . D . Lath ( C) 13 . 2030 . ________________________________________________________ ( 34S) . - . : . . . - - -Child, J. G., 1944, Lower Fars keybeds in the Gachsaran area, Report No. G-647. -Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108-121. -Folk, R.L., 1962, Spectral subdivision of limestone types, in Ham, W.E., ed., Classification of Carbonate Rocks, American Association of Petroleum Geologists Memoir 1, p. 62-84. -Slinger, F. C. P., 1948, Agha Jari lower fars keybeds, Report No. G-721. -Slinger, F. C. P., 1949, The Agha Jari cap rock, Report No. G-751. -Stone, A.J., Watson, S. E., 1962, Agha Jari lower Fars keybeds correlation, Report No. G- 1014. -Strong, M. W., 1939, Notes on petrography and correlation of the lower fars chemical series of Iran, Report No. G-626. -Watson, S. E., 1960, Rivision of the lower Fars key beds in Gachsaran field, Report No. G-946. 2031