AB - ABSTRACT: In Colombia the areas of greatest petroleum exploration interest are characterized by complex structure with folds and thrust faults where it is important to estimate the thermal effects of faults to evaluate organic matter maturity and hydrocarbon generation process. Except for a software model recently developed by the French Petroleum Institute (Francois Roure Personal communication), current and public domain one dimension models do not consider lateral or downward heat flow, which are of normal occurrence in these areas, in additions such models assume instantaneous fault movement contrary to geologic observations. For balanced cross sections with (1) fault bend folds and (2) fault propagation folds we model heat conduction from the basement and organic matter maturity; outputs of the model are temperature and organic matter maturity at each point of the sections at different times while deformation is occurring. Boundary conditions are; absence of lateral heat flow on the lateral border of the sections, constant heat flow at the bottom of the sections, and constant temperature at the surface of the earth. The heat conduction equation applied to the sections was solved by the finite element numerical method with the ANSYS software. The obtained results for different deformation rates are shown and analyzed. Results show lateral variations of temperature and organic matter maturity. Contrary to one dimensional models, the steady state in two dimensions is reached faster, it is cooler and the temperature inversion is shorter. The less deformation rate the less thermal anomaly. At the topographic peaks temperature and maturity are lower than in the valleys. The fault propagation section presents a depth maturity inversion more pronounced than in the fault bend fold model. Depth maturity inversion intensity depends on the ratio between the deformation time T (sub d) and the time passed just after deformation ceased T (sub p) :T (sub d) /T (sub p) . If this ratio is greater the depth maturity inversion will be greater. This paper shows the possibility to determine the organic matter distribution in a structural section with a known thermal and deformation history.
AB - ABSTRACT: The chemical composition of 25 crude oils from Tertiary reservoirs and 12 rock extracts from five organic-rich Cretaceous formations in the Middle Magdalena Basin, Colombia were studied in detail by geochemical methods in order to understand their genetic relationships. The oils have been geochemically classified into four main groups based on the sulfur content, pristane/phytane, dibenzothiophene/phenanthrene, concentrations of oleanane and terpane distributions. Each group occur in different geographic locations. Oils were mainly derived from calcareous, siliciclastic and mixture of these two facies of the Upper Cretaceous La Luna Formation. source-oil correlation is supported by sterane and terpane distributions of and carbon isotope ratios. Some oils in the eastern margin contain relatively higher concentrations of higher plant indicators than the remaining oil samples. The molecular compositions of the oils observed in this study appear to be consistent with the inferred depositional sequence of anoxic marine/pelagic carbonate facies in the north/western sector of the Middle Magdalena, while the eastern sector received a higher proportion of clastic input. This documentation supports the interpretation that the Middle Magdalena oils have been generated "locally", as opposed to have migrated from the region of the Eastern Cordillera. Biomarker maturity parameters indicate that the majority of oils were generated in the early thermal maturity oil window except the Colorado-38 oil which was generated in the middle thermal maturity oil window. Oil maturity data also supports the short migration distances of oils from the early-mature source rocks to the reservoir rocks. The composition of same oils (e.g., La Cira, infants, Conde and Bonanza) is unusual is that gas chromatographic data contains an n-alkane and isoprenoid distribution normally associated with moderately degraded oils, but they also contain a relatively high abundance of gasoline hydrocarbons. This particular gas chromatographic pattern may be explained by the addition of migrant gas/condensate to the "in situ" biodegraded oils. Secondary condensates may also have charged the less biodegraded oils (e.g., Cantagallo-23 and Casabe-471), which could explain the "V" pattern of the n-alkane distribution. This phenomena is observed in productive basins in other parts of the world. The range of oil gravities measured in the Middle Magdalena Basin is attributed to organic facies variations, different levels of biodegradation and late charge of condensate rather than to thermal maturity and migration-related effects.
AB - ABSTRACT: About 1.600 m of core from two major oil fields, and 600 m of outcrop sections near to one of the fields, integrated with thousands of core, side-wall core and outcrop porosity and permeability measurements show that petrophysical properties and reservoir continuity change consistently with the Accommodation to Sediment Supply (A/S) ratio. Channel sandstones exhibit regular, recurring motifs that are associated with changes in A/S conditions. Manifesting the lowest accommodation conditions are amalgamated channel sandstones, up to 5-m-thick, with intraclast-rich bases copped by up to 1-m-thick floodplain mudstones. AT higher A/S conditions, slightly amalgamated channel sandstones have lateral accretion surfaces and are capped by a thicker unit of floodplain mudstones. Channel sandstones in the highest A/S conditions are single storied, possess conspicuous lateral accretion surfaces with thick mud drapes, and have a thick cover of overbank and floodplain deposits. Detailed well log correlations, oil productions, oil production and pressure data support that the former are more lateral continuous while the latter form laterally discontinuous bodies embedded in floodplain mudstones. Petrophysical properties are closely associated with subtle variations in facies, particularly degree of preservation of original bedforms, and both are stratigraphically sensitive. Petrophysical properties of identical sedimentological facies change regularly as a function of its stratigraphic position because of variations in the rates of accumulation and degree of preservation of the sediments. In the case of fluvial strata deposited during an increase in A/S, porosity and permeability are highest in trough cross-stratified sandstones immediately above channel scour bases, and decrease upward to the next scour base. Successive channel sandstones within the same stratigraphic sequence and channel sandstones from one sequence to the next have progressively lower porosity and permeability values in an overall increase in the A/S. An inverse trend is observed during a decrease in A/S.
AB - ABSTRACT: Patterns of porosity in sandstones of the Monserrate Formation (Upper Magdalena Valley) exposed in polished blacks have been digitally recorded using an image processor coupled to a scanning electron microscope operated in backscatter electron mode. Additionally, porosity, permeability and response to mercury injection-capillary pressure tests were measured on some of the imaged samples. Porosity patterns were evaluated via an erosion/dilation-differencing image-processing algorithm and then classified by the self-training classifier, SAWVEC. Changer in the resulting pore type proportions were strongly associated with changes in the mercury porosimetry curves. From the image processing data, five pore types, sufficient to include all of the variability in size and shape of the patterns of porosity, were identified. Variations in the number of pores of each type per unit cross sectional area were related to variations in permeability. The resultant relationships with mercury porosimetry demonstrated that pores of the same type tend to form microcircuits characterized by a limited throat size range. Permeability modeling showed that intergranular pore Types 2 and 4 (secondary porosity resulting from carbonate dissolution) are responsible for permeability in the 0.01-0.10 Darcy range. Type 5 pores (large molds) slightly contribute to permeability, expect in coarse grained rocks where they are efficiently connected by microfractures.
AB - ABSTRACT: Biomarker fingerprinting of 20 crude oils from Putumayo Basin, Colombia, shows a vertical segregation of oil families. The Lower Cretaceous reservoirs (Caballos and "U" Villeta sands) contain oils that come from a mixture of marine and terrestrial organic matter, deposited in a marginal, "oxic" marine setting. The Upper Cretaceous ("T" and "N" sands) and Tertiary reservoirs contain oils with marine algal input deposited in a reducing, carbonate-rich environment. Lithology, environmental conditions and organic matter type of source rocks as predicted from oil biomarker differences correspond to organic composition of two Cretaceous source rocks. Vertical heterogeneity in the oils, even those from single wells, suggests the presence of two isolated petroleum systems. Hydrocarbons from Lower Cretaceous source rocks charged Lower Cretaceous reservoirs whereas oils from Upper Cretaceous source rocks charged Upper Cretaceous and Tertiary reservoirs. Oil migration from mature source rocks into multiple reservoirs has been stratigraphically updip along the "regional" sandstone units and vertical migration through faults has been limited. Biomarker maturity parameters indicate that all oils were generated from early thermal maturity oil window.
AB - ABSTRACT: Spectral analyses of depositional and geochemical time series were conducted on three stratigraphic sections of the Cretaceous Villeta Group, Colombia. Results show statistically-valid cyclicity in distal parasequence stacking patterns and in geochemical variations. Cycles are in the low to high frequency spectra (500 ky-20 ky). Timing of cyclicity suggests climatic controls on deposition, probably caused by Milankovitch orbital oscillations. Stratigraphic intervals enriched in total organic matter or organic matter indicators such as barium coincide with condensed intervals that are generally associated with observable high-frequency cyclicity. Very thin stratigraphic cycles (<0,4 m) contain moderate amounts of organic matter because condensation allows organisms to recycle organic carbon. Condensation also allows time for gradual oxidation of organic matter. Thin to moderately thick (0,6-5,0 m) cycles show the highest quantity of organic matter. Sedimentation rates are high enough to inhibit biological destruction and low enough not to dilute organic carbon within the sediments. Thick cycles are characterized by low organic matter because sedimentation rate dilutes organic material. Organic matter content not only depends on sedimentation rate but also on paleoproductivity and on grain size and shape. The model presented shows how cycle thickness trends may be used as a very simple approach to predict the amount of total organic carbon.
AB - ABSTRACT: A rich assemblage of foraminifers and ostracods recovered from the Guadalupe Group and the base of the Guaduas Formation in the Tausa section are described and illustrated. The Guadalupe Group is characterized by a low microfaunal diversity dominated by the buliminids Siphogenerinoides ewaldi and Siphogenerinoides bramlettei. Increments in microfaunal diversity appear to correspond to the middle part of Transgressive System Tracts. Siphogenerinoides bramlettei (late Maastrichtian in age) supersedes Siphogenerinoides ewaldi (late Campanian in age); the occurrence of the two taxa corresponds to the first sequence boundary in the Plaeners Formation of the Guadalupe Group. The first occurrence of S. bramlettei appears to be "isochronous" in northern South America. The base of the Guaduas Formation is characterized by a high diversity of calcareous benthonic foraminifera (rotaliids). Microfossils from the Guadalupe Group and Guaduas Formation were affected by two taphonomic processes: (1) kaolinite infilling previous to dissolution of tests during the mesodiagenetic stage, and (2) pyrite infilling and pyritization (in sulfidic microenvironments) previous to dissolution of tests during the eodiagenetic stage.