Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300‐725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300‐400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal‐A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal‐A between 600 and 700 m sub‐bottom depth. Inorganic opal‐A is rapidly transformed by crystal growth to opal‐CT. The result is formation of silica cemented mudstone and porcelanite beds. A regional acoustic reflector (called the bottom‐simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal‐A is transformed to opal‐CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub‐bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal‐A to opal‐CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°‐50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary. X‐ray diffractograms and SEM photographs show three silica phases, biogenic opal‐A, inorganic opal‐A’, and opal‐CT. These have crystallite sizes of 11‐16 A, 20‐27 A, and 40‐81 A, respectively, normal to 101. The d(101) reflection of opal‐CT decreases with depth of burial at DSDP Site 192. This occurs by solid‐state ordering and requires at least 700 m of burial. Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.

Forty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high-magnesium calcite and protodolomite, less commonly siderite, rhodochrosite, low-magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty-five per cent of carbonate beds occurred below 400 m sub-bottom depth and 70% in sediment older than 4 m.y. 613C values averaged--l7~20"/,, PDB and 6l80 ranged from 18.59 to 34.1 lo/oo SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO, produced in concert with CH, during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3-5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627-1057 m sections at temperatures of 7-85°C. No apparent calcite precursor of biogenic origin was found for these authigenic carbonates.

Forty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high-magnesium calcite and protodolomite, less commonly siderite, rhodochrosite, low-magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty-five per cent of carbonate beds occurred below 400 m sub-bottom depth and 70% in sediment older than 4 m.y. 613C values averaged--l7~20"/,, PDB and 6l80 ranged from 18.59 to 34.1 lo/oo SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO, produced in concert with CH, during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3-5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627-1057 m sections at temperatures of 7-85°C. No apparent calcite precursor of biogenic origin was found for these authigenic carbonates.

The Rotliegend Lower Leman Sandstone Formation has been studied near the northern limit of its development in Ravenspurn North to determine the influence of structure, sedimentology and diagenesis on reservoir quality. The Lower Leman Sandstone in this area is about 300 ft thick and comprises six depositional cycles which reflect alternating periods of relatively dry and wet climate. Aeolian sands form the best reservoir and formed as small westward-migrating transverse dune fields and draa, at most 60 ft thick. These dunes were frequently reworked by southerly derived, strong fluvial sheetfloods which drained into the Silverpit playa lake.The Ravenspurn area is divided into main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by mid-to late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization.A hierarchy of factors influenced the diagenesis. These were the primary lithological variation, burial history and the structural development of the Ravenspurn North area. Depositional processes also influenced diagenesis; the deposition of allogenic clay and the formation of early quartz, non-ferroan dolomite and anhydrite reduced the reservoir quality of fluvial sheetfloods. Burial diagenesis resulted initially in ferroan dolomite, kaolinite and later quartz precipitation in available primary and secondary porosity. Stable isotope and fluid inclusion studies indicate that ferroan dolomite and later quartz precipitated at about 100°C in Triassic-early Jurassic times from reduced fluids derived partly from the Carboniferous basement. Mixing of these fluids with more alkaline connate water was the main triggering mechanism for authigenic mineral formation.Gas accumulation took place first in the northeasterly B structure which had early closure; the last authigenic phase to precipitate here was poikilotopic siderite at temperatures of about 120°C. Elsewhere diagenetic fluids evolved to a more alkaline state and widespread illitization took place which particularly affected more permeable aeolian facies. The illitization reduced the reservoir quality of the Lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K-Ar dating indicates that peak illitization took place between 150-170 Ma (mid-late Jurassic). Subsequent periods of uplift in the late Cimmerian and particularly during the early Tertiary-Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.

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