Tracing fluid evolution in sedimentary basins with calcite geochemical, isotopic and U-Pb geochronological data: Implications for petroleum and mineral resource accumulation in the Nanpanjiang Basin, South China

Fluid migration in sedimentary basins enable mass and energy transport and play critical roles in geochemical and geodynamical evolution of sedimentary basins. Moreover, reconstructing sedimentary basin fluid evolution from the geological record aids in constraining the evolution of associated petroleum and mineralization systems. As a relict of fluid flow activity, calcite is often a record of fluid flow and therefore can be used to characterize the fluids responsible for its precipitation. Here we study the Nanpanjiang Basin in South China where petroleum reservoirs and Carlin-type gold deposits spatially coincide. Through in situ U-Pb dating and geochemical analysis (87Sr/86Sr, δ18OVienna standard mean ocean water, δ13CVienna Peedee belemnite, rare earth elements) of calcite, this work constrains the key times related to petroleum migration/accumulation and Carlin-type gold mineralization, defines the basin fluid evolution, and proposes a genetic model for petroleum accumulation and gold mineralization within the Nanpanjiang Basin. The U-Pb age (ca. 241.4 Ma) for the gray/black calcite related to bitumen indicates the petroleum migration/accumulation occurred during the Triassic. The U-Pb date (ca. 106−121 Ma) of the white calcite associated with the gold-bearing pyrite, realgar, and fluorite record the lower timing limit of the Carlin-type gold systems. The geochemical data suggest both calcite types are cogenetic but suffered complex evolution with the gray/black calcite precipitating under low temperatures related to the continuous basin burial and the white calcite affected by post formation alteration related to both hydrothermal and meteoric fluids. Combined with the regional tectonic history, the Early Triassic petroleum migration/accumulation and the Early Cretaceous secondary Carlin-type gold mineralization events are considered to be related to the collision between the Indo-China and South China blocks, and the subduction between the Paleo-Pacific and Eurasian plates, respectively.

Application of Biomarkers and Oil Fingerprinting for Genetic Classification of Oil and Prediction of Petroleum Migration Pathways of Aryskum Downfold of South-Torgay Depression

This article presents the results of fingerprinting and biomarker analysis of 254 oil samples derived from 11 different fields and structures in Aryskum downfold of the South-Torgay depression for genetic classification of oils and prediction of petroleum migration pathways. According to the oil fingerprinting results based on patented Shell technology, 12 groups of oils were found: oils in reservoirs of central part of Nuraly field form the first group, while oils in the producing horizons of Western Nuraly, Southern Khayrgeldy, Akshabulak East and fluvial beds of Central Akshabulak fields form the second group. The oils related to the third group were found in the wells exploiting producing horizon I in the north dome of Central Akshabulak, while the oils from wells penetrating lower producing horizons(III-IV-V) of Central Akshabulak, upper producing horizons in South dome of Central Akshabulak and of all producing horizons of the north dome of Akshabulak South constitute the fourth group. The fifth group includes only one oil sample of different genesis from well № 37, which penetrates the paleo-channel №13 at South Akshabulak. The genetic difference of this oil from other oils was also confirmed by its biomarker composition. Most of oil fingerprinting star plots in Aksay field are identical and form the sixth group, although the seventh group comprises only one oil № 47 in Aksay. Sample set with №8 was discovered in the pay zones of Taur field and well № 75, exploiting the same horizon in the northern part of Aksay. The ninth oil group was identified in cretaceous producing layers of the Khairgeldy South-West field and Jurassic beds of the Khargeldy North field, while the identical composition of the cretaceous oil from the Khairgeldy North and Khairgeldy fields forms the tenth group. The last eleventh group includes oil from well №. 12 on South-west Khairgeldy, although it has some similarities with Taur oils. For 20 oil samples was carried out biomarker analysis, according to the results of which all studied oils were formed in terrigenous (shaly) OM, deposited in lacustrine environment. Oils from central Nuraly are more thermally mature and lighter in density than those from western Nuraly. Oils of Akshabulak East are thermally less mature than oils of Central Akshabulak and Akshabulak South despite its deeper deposition. Based on the performed analysis, in the conclusion were presented 5 prospective hydrocarbon accumulation zones for increasing hydrocarbon reserves in the future. But the results of the performed studies provide valuable information only when integrated with confirmed geological and seismic data.

A 3-D geological modeling method and its application to petroleum migration and accumulation simulation

<p>Besides the carrier bed, faults and unconformities are important migration pathways for the 3-D petroleum migration and accumulation simulation. The fault is often ignored or used only as simulated grid boundaries in the traditional 3-D geological modeling, so that the transport function of faults is neglected or weakened.</p><p>In view of the fact that the traditional geological modeling method cannot establish the interrelation of carrier-system (the carrier bed, fault, unconformity, etc.), we propose a hybrid-dimensional mesh modeling technology consisting of body (stratum), surfaces (faults and unconformities), lines and points. The stratum mesh cut by a fault consists of stratum body A, stratum body B and fault surface C. There are two methods: (1) The fault is neglected in the modeling of the geological body, in other words, the mesh form and volume remain unchanged; and (2) The fault is considered in the modeling of the geological body, and the geological body on the two sides of the fault are divided into two parts for modeling. We propose the third processing method. The fault is considered in the modeling of the geological body, and the geological bodies on the two sides of the fault are divided into two parts for modeling, forming stratum meshes. In addition, the fault surface is taken as the third mesh, i.e. surface mesh. At this point, the mesh system is not the original single stratum mesh (3D body mesh) any more, and it also contains the surface mesh (2D surface mesh), therefore it is called a hybrid-dimensional mesh system (hybrid mesh system).</p><p>Based on new hybrid-dimensional mesh of the carrier-system, a special 3-D invasion percolation model (3-DIP) is proposed. The fault transport ability can also be determined by shale gouge ratio (SGR) in the 3-DIP model.</p><p>The new method is applied to the Luliang uplift in Junggar Basin, China, with an area of 3502 km<sup>2</sup>. The strata are composed of Permian - Cretaceous, which are divided into 15 simulated layers. Key simulation parameters of the study area include 2884 plane simulation meshes, 59 faults and 1 unconformity. The total number of formed meshes is 54406, including 45972 body meshes, 7884 surface meshes, 549 line meshes and 1 point mesh.</p><p>The migration pathway of oil is traced by 3-DIP, and the oil accumulation and wax content of crude oil are simulated. By comparing the simulated wax content with the measured wax content, the results are consistent with each other. It is shown that the model is reliable and the results are credible.</p><p><strong>Key words: </strong>geological modeling, migration pathway, hybrid mesh, invasion percolation model, petroleum migration and accumulation simulation, Junggar Basin.</p>