FACIES VARIATION, DIAGENESIS, AND EXPLORATION POTENTIAL OF THE CRETACEOUS RUDIST-BEARING CARBONATES OF THE ARABIAN GULF

The Cretaceous rudist-bearing carbonates of the Arabian Gulf region ar e proven exploration targets for hydrocarbons and form the reservoirs of a number of giant fields, including Bu Hasa, Fateh, Fahud, Idd El S hargi, Rumaila, Shaybah, and Shah. Rudist buildups occur in three prin cipal formations: (1) Aptian Shuaiba, (2) Cenomanian Mishrif, and (3) Maastrichtian Simsima. A regional subaerial unconformity marks the upp er boundary of each of these formations. Associated with the rudists t hat dominate the Shuaiba Formation are calcareous algal crusts, forami nifera, and echinoid plates, which accumulated in mudstone, packstone, and carbonate sands. These rudists are mainly caprinids, with a lesse r number of caprotinids, monopleurids, and requienids, deposited in a normal-marine shallow-shelf setting. The Mishrif Formation contains mo llusk fragments, bioclastic packstones to grainstones, miliolid and no nrudist bivalves in muddy limestones, and rudist (mainly radiolitids a nd caprinids) conglomeratic floatstones, with fragmented rudists mixed with wackestone lithoclasts. The Mishrif sediments accumulated as a p rogradational, low-energy leeward margin formed in marginal slope, sho al-backshoal, and lagoonal settings. The Simsima Formation consists of bioclastic grainstone to packstone, and dolomitic lime mudstones to w ackestone. These are rich in bioclastic grains (Loftusia, rudist and r udist debris, coral, and foraminifera) deposited on a restricted to se mirestricted shallow-marine shelf that was exposed to moderate energy conditions. The excellent reservoir porosity and permeability; of the rudist deposits and their associated sediments are the products of pri mary and secondary diagenesis. Freshwater leaching during post-Aptian, post-Cenomanian, and post-Maastrichtian erosion enhanced the secondar y moldic porosity. Fracturing locally improved porosity and permeabili ty. Other porosity types that occur include interparticle, intrapartic le, vuggy, growth framework, shelter, intercrystalline, and karstic. B ecause of their favorable depositional and postdepositional conditions , the Cretaceous succession of rudists in this region contains many gi ant oil fields.