Greater Burial Depths Research Articles

Hydrocarbon Source Rock Evaluation of Middle Proterozoic Solor Church Formation, North American Mid-Continent Rift System, Rice County, Minnesota

Hydrocarbon source rock evaluation of the Middle Proterozoic Solor Church Formation (Keweenawan Supergroup) as sampled in the Lonsdale 65-1 well, Rice County, Minnesota, shows that: (1) the rocks are organic matter lean (24 of 25 samples have less than 0.8% organic carbon); (2) the organic matter is thermally post-mature, probably near the transition between the wet gas phase of catagenesis and metagenesis (dry gas zone); and (3) the rocks have minimal potential for producing additional hydrocarbons (genetic potential < 0.30 mg HC/g rock). Although no direct evidence exists from which to determine maximum burial depths, the observed thermal maturity of the organic matter requires significantly greater burial depths, a higher geothermal gradient, or both. It is likely, t least on the Saint Croix horst, that thermal maturation of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early phase were probably lost prior to deposition of the overlying Middle Proterozoic Fond du Lac Formation (Keweenawan Supergroup).

Diagenesis and Pore Types of Norphlet Sandstone (Upper Jurassic), Hatters Pond Area, Mobile County, Alabama: ABSTRACT

The Norphlet Sandstone is one of the productive hydrocarbon reservoirs in the Hatteras Pond field. Despite its great burial depth (-18,500 ft or -5,638 m), this sandstone retains fairly good porosity (10 to 14%), but has low permeability (0.1 to 5.0 md). Evaluation of pore-size measurements and pore types helps to explain the low permeability. Pore types are classified on time of generation (primary, secondary), size (megapore, mesopore, micropore), and mode of occurrence (intergranular, intragranular, micropore). Analyses of published data of permeability, porosity, and pore types indicate that intergranular pores produce the best permeability, and that intragranular and micropores provide low permeability. Microporosity is determined empirically as the difference betwee the porosity determined by porosimeter and in thin section. Effective hydraulic pore radius (re) is defined as the radius of a straight capillary pipe having the same permeability as the rock under consideration. Most re values for Norphlet samples indicate the presence of micropores. Observation shows that pores of the water-bearing Norphlet are mostly intergranular and intragranular pores, whereas the hydrocarbon-bearing part has mostly micropores because of extensive illite cementation. Following decementation of carbonate and anhydrite, authigenic illite was precipitated in the sandstone and produced low permeability by two means: (1) by plugging throats of relatively large intergranular pores, and (2) by developing septa that subdivide large intergranular pores into small ones. Principal diagenetic events in the Norphlet were (1) shallow cementation by anhydrite and calcite, (2) decementation, (3) dolomitization and illitization, (4) deep cementation, and (5) stylolitization. Decementation of anhydrite and calcite is probably related to hydrocarbon generation and accumulation and with briny formation water. Authigenic illite developed in secondary pores following decementation and largely controlled reservoir behavior. Differences in reservoir quality in the water-bearing and hydrocarbon-bearing zones are interpreted to be caused by different times of decementation and durations of illite development in the two zones. End_of_Article - Last_Page 1686------------

Structured Water and Its Significance in Primary Oil Migration

ABSTRACT Density of lattice water (mainly OH water) in the shales of the Beaufort Basin is estimated to be approximately 1.4 g/cm3, from the study of the porosity cross plot from neutron and density logs made by Youn (1974). It is interesting to note that this density value is quite close to that of bound or structured water closest to the clay surfaces under laboratory conditions (Martin, 1962). The fact that the viscosity of bound water increases toward clay surfaces suggests that shales must expel more viscous water with increasing compaction. This may be one of the reasons for reduced rate of compaction at greater burial depths. During continuous burial the quality of oil usually improves in the sense that the viscosity decreases. Therefore, at some point during continuous burial history the viscosity of water being expelled from shales may exceed that of the generated oil. If this happens, oil may move preferentially to water, thereby creating the conditions for significant primary oil migration.

Sandstone in Lower Cretaceous Helvetiafjellet Formation, Svalbard: Bearing on Reservoir Potential of Barents Shelf

The Helvetiafjellet Formation consists of 50 to 100 m of largely fluvial sandstones and shales deposited throughout Svalbard during a major regression. Sandstones consist chiefly of monocrystalline and polycrystalline quartz, feldspar, chert, and volcanic (basaltic) rock fragments in (1) quartz and quartzose arenites, (2) quartzose lithic arenites, and (3) volcanic arenites. Quartz and quartzose arenites dominate in western Spitsbergen and were derived from a mixed sedimentary-metamorphic terrane in the west. Volcanic arenites are present in Kong Karls Land in the east, and are intercalated with basaltic lava flows which are considered to be the major source for the sandstones. Quartzose lithic arenites in southeast Spitsbergen are a mixture of quartz sands derived from the west and reworked volcanic sands transported from the east. Cementing minerals are variably developed in these sandstones. Quartz overgrowths are dominant in quartz and quartzose arenites. Kaolinite is present in many feldspar-bearing quartz and quartzose arenites, whereas illite is rare. Chlorite is common, together with calcite, in quartzose lithic arenites. Abundant smectite and calcite with minor zeolites are present in the volcanic arenites, which suggests a strong relation between mineralogy of the detrital grains and that of associated cements. Unstable detrital grains released ions to pore waters, which subsequently precipitated clay and calcite cements. Induration increases and maximum porosity decreases from east to west across Svalbard, owing both to higher temperature, caused by progressively greater burial depth and steeper geothe mal gradients, and to tectonic stress in westernmost areas. Early Cretaceous volcanic activity may have provided abundant chemically unstable detritus to basins in the southern part of the Barents Shelf. As a major consequence, clay cements could form during diagenesis, adversely affecting sandstone reservoir parameters. However, the development of secondary dissolution porosity could improve reservoir characteristics of those sandstones which contain chemically unstable minerals such as feldspar grains and calcite cement.

Sound velocities in calcareous oozes and chalks from sonobuoy data: Ontong Java Plateau, western equatorial Pacific

The Ontong Java Plateau, in the western equatorial Pacific Ocean, is blanketed by over 1 km of flatlying, highly stratified calcareous ooze, chalk, and limestone. Wide‐angle reflection data from 17 sonobuoys deployed over the plateau were analyzed for compressional wave velocities in these sediments. No particular velocity horizons stand out as prominent features that can be correlated from one sonobuoy station to the next. When the data from all stations are plotted together, a fairly consistent velocity‐depth relationship is derived: V = 1.56 + 2.97t with the standard error of estimate equal to 0.08, where V is the compressional wave velocity in kilometers per second at depth t and t is the depth below the sea floor in one‐way travel time of sound in seconds. When the velocity data are plotted against depth in kilometers below the sea floor, the velocity gradient is 1.4 s−1, which is considerably greater than that in most marine turbidites, silts, and clays that are buried to 1 km. The linear uniform velocity gradient is consistent with the type of lithology underlying the Ontong Java Plateau as determined by the Deep‐Sea Drilling Project. That is, there are gradual transitions from ooze to chalk to limestone with intervals of the different lithologies interbedded rather than a single abrupt change from ooze to chalk at one level and from chalk to limestone at a deeper level. A slight difference was seen between velocity profiles in calcareous sediments that had been deposited below and those that had been deposited above the lysocline (the depth below which calcareous ooze is markedly corroded). Sublysoclinal sediments have anomalously low acoustic velocities in the upper 200 m of strata but have normal velocities at greater burial depths. These observations suggest that calcareous sediments deposited below the lysocline lithify more slowly after burial than those deposited above the lysocline.

Petroleum Source Beds on Continental Slopes and Rises

Continental slopes commonly are sites of high marine organic productivity and frequently contain reducing bottom conditions, quiet water, and intermediate sedimentation rates, all of which favor deposition of organic-rich sediments. These deposits typically have high percentages of aquatic organic matter with high petroleum yields as contrasted to relatively organic-lean shelf deposits which contain primarily low-yield terrestrial organic matter. Conversion of organic matter in potential source beds to oil and gas requires a combination of temperature and time. These variables are controlled primarily by the geothermal gradient, the rate of burial, and the age of the source interval. Most divergent margins need between 2 and 4 km of overburden for oil generation and from 3 to 7 km for gas generation. Typically cooler and younger convergent margins and deltaic margins must have even greater burial depth to achieve the same results. Continental margins, including present slopes and rises, can contain oil and gas source beds when minimum requirements of organic content, kerogen type, and thermal maturity are met. Migration and accumulation are most efficient, however, where reservoir sequences prograde over source beds in areas of structural complexity. Preservation of trapped petroleum requires effective seals and minimal structural readjustment after accumulation. All these conditions can be found on present slopes and rises, although they are not common, and must be considered as part of any economic evaluation of these largely untested deep-water realms.

The Mid-Atlantic Ridge near 45° N. XI. Seismic velocity, density, and layering of the crust

Compressional wave velocities at pressures to 1000 kg/cm2 and densities are given for a representative suite of rocks selected from 42 dredge hauls on the Mid-Atlantic Ridge near 45° N. The spectrum of rocks studied includes vesicular and massive basalts, metabasalts, meta-gabbros, and serpentinites. Evidence is presented for block faulting of an originally continuously layered crust of vesicular basalt and massive basalt underlain by a metamorphosed basalt and gabbro sequence. A study of the velocities of these layers at in situ pressures shows that they correlate well with the seismic layering of the oceanic crust. The velocity of the massive basalt layer is in agreement with that reported for layer 2. The underlying layer, consisting of low-to-medium grade metamorphosed basalts and gabbros (greenstones and greenschists) exhibits higher velocities. None of these exceed 6 km/s but it is suspected that these rocks at greater burial depths will exhibit velocities comparable to those of layer 3. The occurrence of serpentinites at all elevations on the slopes of seamounts implies that they do not form a continuous layer and because of their relatively low velocity, it is unlikely that layer 3 is composed of these rocks. Their presence as hydrated diapiric intrusives is more plausible.