Geohistory Analysis Research Articles

Relationship Between Eustatic Sea Level Changes and Basement Subsidence

Geohistory reconstructions based on well data provide a direct and quantitative method of estimating basement subsidence. Geohistories, in addition to responding to rates of sediment accumulation and to local and regional subsidence mechanisms, also reflect eustatic sea level changes. We have determined quantitatively that component of subsidence that is common to a worldwide set of wells. After compensating for the response of the crust to sedimentation, we find some correlations between the global rate of subsidence and low-frequency changes in the height of eustatic sea level as determined from the stratigraphic interpretation of seismic data. These results suggest that basement subsidence data, which are independent of the techniques used previously, may be used to re ognize variations in eustatic sea level. However, we are unable to determine the size of the eustatic sea level excursions because of difficulties in determining and separating variations in lithospheric rigidity, thermal subsidence, and sediment supply independent of tectonic subsidence and sea level variation. We compare subsidence curves in an empirical fashion with published eustatic sea level curves, thus avoiding the application of geologic models involving crustal properties we cannot quantify. Geohistory analysis is also a powerful means of studying subsidence on a regional scale and so of deducing the regional tectonic history. We illustrate this point by subsidence maps through time for the North Sea.

Regional Distribution of Hydrocarbon Fluid Inclusions in Carbonate Fracture Filling Cements: Geohistory Analysis and Timing of Oil Migration, Oman Foredeep: ABSTRACT

Fractured, reservoir limestones in Oman and the United Arab Emirates include the Shuaiba (lower Aptian) and Mauddud (upper Aptian-lower Cenomanian). Deposition of these bioturbated, argillaceous foraminiferal-peloidal wackestones and packstones ceased in the Early Cretaceous as the Oman foredeep subsided and filled with pelagic sediment. Petrography and geohistory analysis of four wells and one outcrop suite reveals five stages of diagenesis, fracturing, and fluid migration. (1) Shelf emergence: early cementation associated with regional unconformities overlying both limestones; (2) pre-orogenic shelf emergence, late Cenomanian to Turonian: fractures cutting Stage 1 cements are healed by very cloudy, cleaved, and twinned calcite containing microfractures with yellow-white fluorescent, hydrocarbon fluid inclusions; (3) initial foredeep downwarp of 0 to 800 m (0 to 2,624 ft), Coniacian to early Campanian: fractures crosscutting Stage 2 fractures are healed with cloudy, cleaved, and sometimes twinned calcite containing dull-blue fluorescent, hydrocarbon fluid inclusions; (4) rapid subsidence and filling with 600 to 3,400 m (1,970 to 11,155 ft) of flysch, exotic blocks, and thrust toes, Campanian to Maestrichtian: burial and tectonic stylolites crosscut Stage 2 and 3 fractures; and (5) uplift of the Oman Mountains after 3,900 + m (12,795 + ft) burial by early Tertiary: fractures crosscutting all diagenetic features are filled with clear untwinned and uncleaved calcite containing only non-fluorescent, aqueous fluid inclusions. If we can correlate earliest styl lite formation with a minimum burial load of ~ 800 m (~ 2,625 ft), then the hydrocarbon inclusions in Stage 2 fractures must predate all of Stage 4 and most of Stage 3. In the deepest portions of the foredeep, close to the Oman Mountain front, this limits the presence of oil in fracture porosity to late Turonian-early Campanian time. Farther to the west, in the shallower parts of the foredeep, this constraint relaxes, and oil migration occurred as late as early Tertiary. End_of_Article - Last_Page 434------------

Maturation Modeling in Otway Basin, Australia

The Otway basin is a Jurassic to Pliocene sedimentary basin formed on the southern Australian continental margin. Its formation is associated with rifting and breakup of the Australian and Antarctic plates. Lithospheric cooling and contraction have probably produced post-breakup subsidence. Either lithospheric stretching or deep crustal metamorphism may have produced pre-breakup subsidence. These mechanisms have identifiable thermal histories. Organic diagenesis (specifically the reflectance of vitrinite in oil) is empirically determined by the thermal and depositional history of an organic sediment. Thus, the stages of hydrocarbon maturity of Otway basin sediments can be modeled. Depositional history is determined from geohistory analysis and thermal history depends on the subsidence mechanism applied to the basin. A paleo-heat-flow history derived from the deep crustal metamorphism model of subsidence produces a maturation profile with depth that is consistent with observed vitrinite reflectance data, although organic diagenesis modeling is relatively insensitive to precise details of thermal history. Depositional and maturation history modeling for the present day, 20 Ma ago, 40 Ma ago, and 60 Ma ago is applied to a se smic profile across the southern Australian continental shelf in the Otway basin as a demonstration of the projection backward in time of sedimentation and organic diagenesis.

RECENT ADVANCES IN BURIAL AND THERMAL GEOHISTORY ANALYSIS

Geohistory analysis is a technique for plotting the cumulative subsidence of various chronostratigraphic horizons encountered in an exploration well or section. The intervals between horizons are corrected for sediment compaction and are plotted relative to an estimated ancient seabed (or land surface) as a function of geological time. Speed and precision have been added to this procedure by automatic computer processing and plotting. A variable eustatic sealevel datum has also been introduced. Accurate quantitative assessment of depth sub-seabed (or subsurface) of any horizon may be combined with a model of heatflow history for the well location to compute a maturation integral and theoretical vitrinite reflectance for that horizon as a function of geological time. Comparison of theory with present observations may be used to refine the model heatflow history. Palaeomaturation may be readily assessed.An offshore Perth Basin example is used to illustrate the effects of palaeosealevel, palaeoheatflow and unconformities and hiatuses. Analysis of a cross-section through three wells is used to demonstrate the potential for accurate calculation of a palinspastic section, palaeomaturation and migration direction. Geohistory analysis is demonstrated to be an essential tool in rigorous basin evaluation.

Burial and Thermal Geohistory Analysis at Passive Continental Margins: ABSTRACT

Burial geohistory analysis involves progressive stripping of sediment bodies defined by well chronostratigraphic horizons. Underlying section, to basement, is corrected for compaction effects and paleo-water-depth, and plotted as a paleowell section in a time-depth framework. The procedure is amenable to computer processing. Given a tectonic model for basin subsidence, a theoretical paleobent flow function can be applied to the calculation of integrated thermal and maturation history of the well section as a function time and depth. The procedure may be extended to cross sections providing migration direction relative to trap formation. Our examples, from the southern and southwestern margins of Australia, illustrate these applications, and provide constraints to passive margin formation processes. End_of_Article - Last_Page 966------------

Quantified stratigraphy – an exploration approach for the eighties

Quantified stratigraphic parameters from seismic and well data provide the paleogeographic and facies information necessary for the development of an accurate geologic framework for hydrocarbon exploration. The key parameters are (1) geologic age, (2) sea-level changes, (3) paleobathymetry and paleotopography, (4) subsidence and uplift, and (5) sedimentation rate. This paper discusses how these parameters are quantified from and applied to seismic and well data using seismic stratigraphy and geohistory analysis.

A Jurassic Time Scale

Application of geologic ages expressed in numeric time (millions of years) is a recent but rapidly expanding factor in petroleum exploration (rates of tectonic movements; basin models; geohistory analysis; prediction of reservoir quality, source potential, and HC maturation; correlation of geophysics with geologic events, etc.). This paper proposes an emendation of published versions of the Jurassic time scale. The standard geochronologic subdivision is combined with a linear-numeric scale, with a biostratigraphic scheme (ammonites, calcareous nannofossils), and with the geomagnetic-reversal time scale. To this purpose, basement ages at some Deep Sea Drilling Project sites are evaluated and related to the geomagnetic-reversal scale. The numeric scale certainly cannot be r garded as precisely and firmly determined throughout the Jurassic, but it seems adequate as a broad framework.