GEOLOGIA REGIONAL
Geológicamente, el sureste del departamento de Casanare se encuentra localizado dentro del cinturón plegado y fallado de la cordillera oriental de Colombia, principalmente en el paisaje de piedemonte, donde predominan sedimentos de edad terciaria y cuaternaria limitados por dos sistemas de fallas regionales: El sistema de fallas de Guaicaramo al noroeste y el sistema de fallas de Yopal al sureste. El área se caracteriza por presentar una actividad tectónica intensa con una dinámica compresiva manifiesta por pliegues sinclinales y anticlinales y fallas de cabalgamiento. Cronológicamente, las rocas afloran en fajas mas o menos regulares siguiendo la misma dirección de las principales estructuras del país, es decir, SW –NE. Las rocas mas antiguas afloran hacia la parte alta de la cordillera, o bien hacia el NW del municipio y progresivamente, van apareciendo las secuencias de rocas mas jóvenes en el mismo sentido en que la pendiente disminuye hacia los llanos. Así, los materiales se ubican en una secuencia que se extiende desde el cretáceo hasta el cuaternario, correlacionándose con los distintos paisajes que se han separado; de esta manera, en la montaña predominan materiales del cretáceo, en el piedemonte, lomerio y altiplanicie abundan materiales terciarios, en tanto que la planicie aluvial y los valles están constituidos por sedimentos del cuaternario. La clasificación de las unidades Geológicas presentes en Tauramena, se realizó con base en la litología existente en el área del municipio, siguiendo la nomenclatura utilizada por BPX, la cual es correlacionable con la utilizada por INGEOMINAS en el Piedemonte llanero. (Plano ZFBT1003).
Estratigrafía
The sedimentary stacking pattern registered corresponds to several cycles of eustatic variations, so that it was possible to identify high energy fluvial sediments of Une Formation laying discordantly on Paleozoic rocks, a subsequent rising in sea level allowed the accumulation of Gacheta Formation, followed by coastal deltaic sediments of Guadalupe Formation in which Coastal-Fluvial sediments of Mirador Formation lies discordantly, covered by a thick sequence of estuarine sediments composed by shales and sandstones of Carbonera Formation .
The lower Miocene marine transgression was identified by a thick sedimentary pile composed by the shales of Leon Formation, Finally it is possible to characterize the basin colmatation with the sediments of Guayabo Formation.
GUAYABO FORMATION
Guayabo Formation is a siliciclastic sequence composed by sublithic arenites interbedded with conclomeratic arenites toward the top , siltstones and varicolored claystones , the conglomeratic arenites are common toward the the top , while the clay content increases progressively toward the base. Upon this characteristic Guayabo Formation can be divided in two intervals, that correspond to a change in the sedimentary conditions, this change is evident upon the clay amount and coloration, the intervals and physical characteristics of the rocks are described as follows.
Interval A from 50ft MD; 50TVD to 4200 MD; 3898.97 TVD.
This interval is composed mainly by thick intercalations of sublithic sands, sandtones, some conglomeratic sands, varicolored claystones and siltstones.
The remaining portion of the components corresponds to several sorts of lithics, which at a basic glance correspond to sedimentary rocks (Chert, Siltstones, Sandstones), and metamorphic rocks (Quartzites and green Schist). The conglomeratic sands associated, often has the same composition, so that it presumably belongs to orthoconglomerates common of channel bars.
Throughout the whole interval it is possible to notice some thin intervals with iron oxides abundance – hematite, and limonite, this horizons have intense red to reddish orange colorations, which may suggest pedogenic processes.
Interval B from 4200ft MD; 3898.97TVD to 5427’ MD; 5210.42 TVD.
This interval is composed mainly by varicolored claystones interbedded with reddish siltstones and fine grained sandstones in minor proportion.
Leon Formation top was defined by the gradual occurence of greenish shales, also one of the lithological markers that points out Guayabo and Leon contact, is the greenish colorations of the basal claystones of Guayabo Formation. Along with the shale increase; appear abundant traces of pyrite. This lithological change is supported by an increase in gamma ray values, showing a positive inflection of the curve, compared to the uppermost sands and siltstones of Guayabo Formation.
Leon Formation corresponds to a homogeneous succession of greenish Shales, interbedded with gray claystones and reddish siltones toward the upper contact, becoming shale monotonous toward the bottom.
Throughout the whole sequence can be seen siderite nodules, pyrite traces, and coal thin beds.
Upper contact with the overlying Leon Formation is sharp, and was defined by a sudden occurrence of sands; this lithological change is evident also in the response of gamma ray curve, with a strong negative inflexion; that allows pointing out the top of Carbonera Formation.
Carbonera Formation can be divided into 8 units according to the lithological composition; these changes reveal variations in the sedimentary conditions within an estuarine depositional system. So that Carbonera Formation Units have been named from top to base as C1; C2; C3; C4; C5; C6; C7 and C8 . Each one is described as follows.
As mentioned above C1 Unit upper contact is characterized by the rapid occurrence of quartzose sands. This Unit is composed mainly by sands, interbedded in minor proportion with greenish gray claystones, greenish shales, gray siltstones and occasionally and toward the bottom, coal thin beds.
Throughout the unit it is common pyrite, organic material, and iron oxides as traces.
Carbonera C2 Unit top was defined by a gradual increase in shale and claystone content toward the C1 Unit bottom, as well as the decrease in sand fraction, this lithological change is supported by the correspondent increase in gamma ray values and the decline in resistivity, as well as the tendency of the resistivity curve, from wide amplitudes throughout C1 to thin amplitudes toward the entrance and along of C2.
Carbonera C2 Unit is composed mainly by greenish shales, interbedded with grayish claystones, grayish siltstones, occasionally sand and sandstones.
CARBONERA FORMATION – C3 UNIT
Lithologically the contact between Carbonera C2 and C3 Units is marked by the occurrence of predominantly sandy intervals, this change has an evident response in gamma ray values with a general decrease in comparison with those values registered in C2, accompanied with the the rise in resistivity and the amplitude increase for resistivity curves, which supports the lithological change at the C3 and C2 contact.
The general composition of C3 is quite similar to C2, what is noticeable is the reddish colorations for claystones and siltstones, and the occurrence of iron oxides, which suggest sub aerial processes and basin shallowing. Claystones and Siltones are interbedded with greenish shales which are predominant along C3. Sporadically there are sandstones toward the middle of the unit and siltstones become sandy.
The common mineral traces along C3 Unit are Pyrite, Dolomite and Coal.
Carbonera C4 Unit top was defined lithologically by the progressive increase in shale content , along with the occurrence of brownish organic rich shales, this lithological change is observed in the resistivity curve; with the drop off of the values in comparison with C3 Unit, the contrary effect happens in gamma ray values, with the sudden increase characteristic in shales. These parameters go together with a progressive decrease in background gas, which allowed pointing out C4 Unit entrance.
Through the Unit are common traces of Coal, Pyrite and Siderite nodules.
CARBONERA FORMATION - C5 UNIT
Carbonera C5 Unit top was defined by the occurrence of very fine grained sandstones and sands; as well as the increase in resistivity; and the sharp decrease in gamma ray.
Carbonera C5 Unit is composed by sands and sandstones interbedded with shales and claystones toward the top and the bottom, this sandy intervals are separated by a thick clayey succession. It is remarkable the coloration of the clays that often characterizes the lower part of C5 Unit, also suggesting the basin shallowing.
Carbonera C6 Unit top is evident upon the increase in shale content, as well as the lack of sand common toward C5 bottom, this lithological change is seen in LWD well logs, as the increase in gamma ray accompanied by the decrease in resistivity and the curve amplitude, correspondent to a shale predominant succession.
Carbonera C6 Unit is composed by greenish shales, interbedded with grayish claystones and siltsones in minor propotion.
Carbonera C7 Unit contact was defined by the occurrence of fine grained quatz sands, along with the decrease in gamma ray and the correspondent increase in resistivity .
C7 Unit is composed by grayish claystones, interbedded with shales, sands and siltstones in minor proportion.
Carbonera C8 Unit top was defined by the lack of sand in comparison with C7 Unit, also the occurrence of dark gray shales toward the contact; the lithological changes are reflected in the decrease in resistivity and the positive inflection of gamma ray curve.
Carbonera C8 Unit is composed by varicolored claystones interbedded with shales and dark siltsones in minor proportions.
Pyrite, and carbonaceous material are common as traces throughout the whole interval.
The Mirador Formation constitutes one of the main objective of the well. It occupies a place privileged into the lithostratigraphic column of the llanos basin. It is one of the main producers of liquid hydrocarbons.
This Formation was defined by the increase in Shallow and Deep Resistivity values, decrease of Gamma Ray values and occurence of Sand. It is composed mainly by Sand with thin intercalations of Shales and Claystones. Along the whole drilled section; appear carbonaceous material wich is evident with the increase in the background gas. It is frecuent to find throughout this unit traces of Pyrite.
Remarks:
SAND and SANDSTONE oil show from 9580 feet to 9610 feet.
The Mirador Shale Unit was defined by the increase of Shale, increase of Gamma Ray values and decrease in Shallow and Deep Resistivity . It is composed by Shale with thin intercalations of Sandstones, Sands and Claystones, throughout the drilled section appear Pyrite traces.
LOWER MIRADOR UNIT FORMATION
The Lower Mirador Formation was defined by the increase of Sand, Sandstone, decrease of Gamma Ray values and increase in Shallow and Deep Resistivity . It is composed by Sand with intercalations of thin beds of Sandstone, Claystone and Shale. Pyrite appear as traces along the whole drilled section.
Remarks:
Sand and occasional Sandstone oil show from 9760 feet to 9820 feet.
The transitional contact with the overlying Lower Mirador Unit is marked by the occurence of Carbonoceous Silstones, along with the increase of Shale and the change in the grain size of sands to predominatly coarse to very coarse grain.
Guadalupe Shale Unit is composed by intercalations of Shale with thin layers of Silstones, Claystones, Sandstones and Sands.
Remarks:
Sand and Sandstone oil shows from 9820 feet to 9840 feet.
The Guadalupe Formation constitutes the main objective of the well because of the hydrocarbons content. It is composed mainly by Sandstones and Sands with intercalations of Claystones. Toward the bottom are common thin levels of Carbonoceous Siltstones and Shales. The top was defined by the increase of Sandstone, the decrease of Gamma Ray values and the increase in chromatography.
Remarks:
SAND and SANDSTONE oil shows from (9848 feet to 9860 feet) & (9870 feet to 9880 feet).
SAND and SANDSTONE oil shows from 9860 feet to 9870 feet.
The Gacheta Formation is represented by a sequence of Shales and Claystones with thin intercalations of Siltstones, Sandstones and Sands levels Toward the base the Formation is predominantly sandy. The Gacheta Formation top is marked by the increase of dark grayish Shales , along with the change in the grain size of sand to predominatly fine to very fine grain, as well as the increase of Gamma Ray values.
GACHETA SST FORMATION
The Gacheta Sandstone Formation was divided into two intervals. The first interval (from 9982 feet to 10050 feet) is composed by potent Sandstones beds interbedded with Shales, Claystones layers (predominatly toward the base), and very thin Carbonaceous Silstones. Towards the middle of the interval there are some very poor oil shows.
The second interval (from 10050 feet to 10146 feet) is composed mainly by Claystones with intercalations of Shale, Sandstone and in minor proportion Sands and Siltstones.
Remarks:
Sandstone oil shows from ( 9980 feet to 9990 feet ) and (10010 feet to 10030 feet).
Sandstones oil shows from (9980 feet to 9990 feet).
Sandstones oil shows from (9990 feet to 10000 feet).
Sandstones oil shows from (10000 feet to 10010 feet ) and (10010 feet to 10040 feet).
The Une Formation constitutes one of the main objectives of the well. It is composed mainly by Sandstone and Sands with intercalations of Shales and Claystones , and sporadic thin levels of Siltstone toward the lower part of the drilled section.
Remarks:
Sandstone oil shows from ( 10140 feet to 10180 feet ) is in TRACES.
PALEOZOIC FORMATION
Paleozoic rocks correspond to Sand and consolidate Sandstones, interbedded with thin layers of Claystones, Shales and esporadic thin layers of Siltstones. Along all drilled section appear Pyrite as traces.