Gucheng Area Research Articles

Reservoir forming conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng area, Tarim Basin

The Gucheng region is oil and gas accumulation area of Tarim Basin，dominated by beach controlled lithologic oil and gas reservoirs，which has superior oil and gas accumulation conditions and broad exploration prospects. The lower Paleozoic strata in the Gucheng region are the Carboniferous, Ordovician, and Cambrian systems from top to bottom. The Silurian, Devonian, and Permian systems are missing. The lower Ordovician – Cambrian are the main exploration target layers in the area. There are four reservoirs: the Ordovician group, the Yingshan group, the Penglaiba group and the Cambrian group in The Gucheng region, The upper part of the Yijianfang formation and Yingshan formation are limestone reservoir, which is a set of potential reservoir. The lower part of the Yingshan formation, the Penglaiba formation, and the Cambrian formation developed three sets of dolomite reservoirs. The distribution of oil and gas in Gucheng region was mainly found in Yijianfang formation, Yingshan formation, Penglaiba formation and Cambrian of the Ordovician. The Yingshan group and the Hanwu group were the main exploration target layers. Yingshan formation and Cambrian are the main exploration targets, and Yijianfang formation and Penglaiba formation of Ordovician can be used as the concurrent exploration targets. Gucheng region can form two major realistic exploration areas: dolomite reservoir of Ordovician and reef beach of Cambrian. The Ordovician dolomite reservoirs are mainly searched for the inner shoal of the platform edge high-energy facies belt, and the Cambrian system is mainly search for the high-energy reef beach far away from the platform edge facies belt in the north of cherchen fault.

Hydrothermal dolomite reservoirs in a fault system and the factors controlling reservoir formation-A case study of Lower Paleozoic carbonate reservoirs in the Gucheng area, Tarim Basin

The widespread hydrothermal activity in the Lower Paleozoic carbonate reservoirs of the Tarim Basin has received much attention. The discovery of hydrothermal dolomite reservoirs indicates that hydrothermal alteration may have a positive effect on reservoir development. However, the factors controlling hydrothermal dolomite reservoirs and their pore development characteristics are difficult to recognize and interpret. Based on the lithology, petrology, pore fabrics and geochemical characteristics of the Lower Paleozoic carbonate reservoirs in the Gucheng area, this research discloses that the origin of the carbonate reservoirs is related to alteration by hydrothermal brine. Further, hydrothermal dolomite reservoirs are strongly heterogeneous in different fault zone structures. This research divides the fault structures into fault cores (type 1) and damage zones (types 2 and 3). Type 1 is characterized by many breccias cemented by saddle dolomite, locally developed vugs or fractures in the cores, low density (1.7–2.8 g/cm3) and resistance (22.4–1075 Ω) in the log response, and shows abundant black-spotted vugs and sinusoidal fractures in formation microscanner images (FMI). Due to poor connectivity and high heterogeneity (permeability: 3.24 to 0.01 mD, average 0.59 mD; porosity: 5.1 to 0.7%, average 1.8%), type 1 hydrothermal dolomite reservoirs show both positive and negative hydrothermal fluid contributions. In the damage zones, especially in the areas near fault cores, type 2 is rich in fracture-pore systems in both the cores and FMI and is characterized by serrated fluctuations in acoustic waves and densities (2.4–2.8 g/cm3). Type2 performs well in terms of physical properties (permeability: 8.06 to 0.011 mD, average 0.837 mD; porosity: 6.6 to 0.65%, average 2.5%), indicating potentially favorable areas for the development of hydrothermal dolomite reservoirs. At the distal end of a damage zone or in poorly developed areas, type 3 features filling by hydrothermal products or higher densification (permeability: 0.77 to 0.0003 mD, average 0.125 mD; porosity: 1.6 to 0.2%, average 0.2%). The well logging response of type3, which has a density of around 2.8 g/cm3 and flat acoustic wave curves, is similar to that of the host rock. The differences in the development of hydrothermal dolomite reservoirs among diverse fault zones suggest that the damage zone (type 2) may be a potential area for hydrothermal dolomite reservoirs in the fault system. This study illustrates a model in which the hydrothermal fluid dissolves host rock in the proximal area and precipitates it at the distal end of the damage zone. The hydrothermal dolomite reservoir characteristics of different fault zones are also further interpreted and described. Therefore, different locations within a fault zone need to be considered and distinguished in studies of the factors controlling hydrothermal dolomite reservoirs in fault systems. This view may provide new ideas regarding whether hydrothermal activity in the Tarim Basin plays a positive role in the formation of reservoirs and offers new suggestions for the exploration of hydrothermal dolomite reservoirs in fault systems.

Characteristics and pore genesis of dolomite in Ordovician Yingshan Formation in Gucheng area, Tarim Basin

Characteristics and pore genesis of dolomite in Ordovician Yingshan Formation in Gucheng area, Tarim Basin

Petroleum geological conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng Area, Tarim Basin, China

Abstract Lower Paleozoic carbonate rocks are an important exploration area in craton area of the Tarim Basin, with the proven oil and gas reserves of more than 2.2×108 t, but no large-scale discovery has been made in the Gucheng area so far. The key issues restricting exploration are that the source rock, reservoir scale and law of oil and gas enrichment are unclear. By systematically examining the petroleum geological conditions of Lower Paleozoic carbonate rocks, the following findings are reached: (1) Source rocks of slope-basin facies developed in Cambrian-Lower Ordovician in the Gucheng area. (2) The dolomitized beach in the lower part of Ordovician Yingshan Formation has large-scale reservoirs, good reservoir-cap assemblage and developed gas source faults, and is an important field for increasing reserves and production in the near future; hydrocarbon enrichment is controlled by reservoir and gas source faults, and the central dolomitized beach zone is the main exploration area. (3) The Cambrian platform margin reef beach, large in scale, good in physical properties and close to source rocks, has the possibility to form monolithic gas field; the caprock and preservation conditions are the key factors for hydrocarbon enrichment; the northern part of the phases I and II platform margin reefs has better sealing conditions, and is the main direction of next exploration. (4) Limestone fault solution reservoirs in the upper part of Ordovician Yingshan Formation, controlled by faults and small in scale, but good in reservoir-cap combination, worth exploring. (5) The granular limestone beach of Ordovician Yijianfang Formation is well developed and gas-bearing, but short in exposure dissolution time, and the reservoirs are strongly heterogeneous, and are a potential exploration field.

Experimental analysis on heating of oil inclusion in Gucheng area of East Tarim

In this paper, the formation and preservation conditions of crude oil cracking gas reservoirs are analyzed by heating experiments of oil inclusions. The study of tectonic development history shows that Gucheng low uplift mainly undergoes three evolutionary stages: early extension, middle uplift and late subsidence. Up to the end of Devonian and early Hercynian, the southeastern Altun compression resulted in warping plate uplift and denudation in Gucheng area, with denudation thickness of about 1000-2800m.The experimental results show that no inclusion rupture occurs during the whole process of heating to 600?.It was observed that the phase state of inclusions changed obviously during heating process, and the fluorescence of inclusions weakened obviously after natural cooling, reflecting the cracking process of crude oil. The results of high resolution laser-raman spectrum measurements of oil inclusions after heating experiments show that methane is the main component of gas inclusions, while carbon dioxide is small. According to the microscopic comparison of oil inclusions before and after heating, it is inferred that the sample has not undergone this high temperature process. Theoretically, it is assumed that oil and gas inclusions will rupture when heated to a certain temperature. It is possible that this experiment did not reach the limit temperature of inclusion rupture.

Comprehensive insight of the Cambrian carbonate platform types as well as margin segmentation characteristics' exploration in Tarim Basin, China

The carbonate platform types and features of the platform margin belt plays an important role in controlling the reservoir formation; it also affects the relationship between reservoirs and caps. The Cambrian carbonate platform in the Tarim Basin underwent three evolutionary processes, namely, the Early Cambrian ramp platform, the Middle Cambrian edging evaporative platform, and the Late Cambrian edging platform; the northern platform margin was the deposition type, whereas the eastern platform margin was for fault control, additionally, the Lungu-Gucheng had evident sectional differences. The line from Wushi-Kashi-Maigaiti-Hetian to Minfeng of the southwestern Tarim Basin was an ancient land in the Early Cambrian. Through evolution, the sea level raised the underwater lows in the Middle and Late Cambrian period that possibly developed it to platform edge deposits in the Late Cambrian. The carbonate platform margin of both steep slope and gentle slope formed different reservoir-seal assemblages. The Upper Cambrian aggradation-progradation platform margin reservoir in the steep slope of the eastern Lungu and Gucheng area was developed with good connectivity, its caprock had been always the key of the platform margin reservoir-seal assemblages. Therefore, the reef-beach located behind the platform margin belt near the seaside of the lagoon had favorable reservoirs; the reservoirs often overlaid carbonate caprock which formed good reservoir-seal assemblages. The platform margin belt in the gentle slope in the Well Yingmai 7-Well Yingmai 8, in west Tabei, was a dolomite reservoir for algal mound and reef-bank complex with caprock of middle-lower Cambrian dolomicrite, gypsum dolomite, and mud dolomite. Aforementioned dense layers' reservoir-seal assemblage was superior to that of the eastern Lungu and Gucheng that had better exploration prospects.

Ordovician gas exploration breakthrough in the Gucheng lower uplift of the Tarim Basin and its enlightenment

A steady high yield natural gas flow was tapped in the Ordovician strata of Well Gucheng 6 drilled in the Gucheng lower uplift in the Tarim Basin in 2012, marking the discovery of another carbonate gas exploration field after the other two fields in the middle and northern Tarim Basin (the so called Tazhong and Tabei in the industry). The exploration in the Gucheng lower uplift has experienced three stages: the first stage, marine facies clastic exploration from 1995 to 2003, focusing on the Devonian Donghe sandstone lithologic traps, the Silurian overlapping lithologic traps, and the Upper Ordovician shelf slope turbidites; the second stage focusing on the reef shoal carbonate reservoirs from 2003 to 2006, during which oil and gas were first discovered in Well Gucheng 4; the third stage can be divided into two periods, in the first period, deeper insight into interbed karstification reservoir exploration, intense research on tricky seismic issues, selection of favorable zones, and 3D seismic deployment in advance laid a robust foundation for breakthroughs in oil and gas exploration; and during 2009–2012, through an in-depth investigation, Well Gucheng 6 was drilled, bringing about the major breakthrough in oil and gas exploration in this study area. This success proves that the Lower Paleozoic carbonate rocks in the Gucheng area have good geological conditions and broad prospect for oil and gas exploration, which give us enlightenment in three aspects: a. new insight into geologic understanding is the prerequisite of exploration breakthrough; b. addressing bottleneck technologies, and acquiring 3D seismic data are the guarantees of exploration breakthrough; and c. emancipation of mind and persistent exploration are key to the findings in new domains.

Study on hydrocarbon-generation evolution and prediction of favorable exploration areas in Gucheng area of Tarim basin

Lower Ordovician and Cambrian are the good source rocks in Gucheng area compared to the other areas in the east of Tarim Basin. During the early of Caledonian movement after Upper Ordovician deposited, the source rocks of Lower Ordovician quickly went into gate of Hydrocarbon-generation and generated oil & gas. Because the time was so short and buried depth was so deep, the generated oil is not able to be expulsed from source rocks and they had been cracked to generated gas along with source rocks, which Hydrocarbon-generation didn’t stop until the source rocks arrived at the dead line of natural gas generated. In the following tectonic movements, the buried depth and temperature didn’t measure up to the conditions and the thermal evolution of source rocks had reached the dead line of natural gas generation after Caledonian movement, and no gas could generate. Used SPI, we conclude that the forming pattern of reservoirs is self-generating and self-preserving based on analysis of lithologic characteristics. The favorable exploration areas are located in Paleo-accumulation in the Palaeozoic group, especially dolomite reservoir in Cambrian.