Organic Geochemical and Petrographic Characteristics of the Coal Measure Source Rocks of Pinghu Formation in the Xihu Sag of the East China Sea Shelf Basin: Implications for Coal Measure Gas Potential

Abstract

Coal measure source rocks, located in the Xihu Sag of the East China Sea Shelf Basin, were analyzed to define the hydrocarbon generation potential, organic geochemistry/petrology characteristics, and coal preservation conditions. The Pinghu source rocks in the Xihu Sag are mainly gas‐prone accompany with condensate oil generation. The coals and shales of the Pinghu Formation are classified from “fair” to “excellent” source rocks with total organic carbon (TOC) contents ranging from 25.2% to 77.2% and 1.29% to 20.9%, respectively. The coals are richer in TOC and S1+S2 than the shales, indicating that the coals have more generation potential per unit mass. Moreover, the kerogen type of the organic matter consists of types II‐III and III, which the maturity Ro ranges from 0.59% to 0.83%. Petrographically, the coals and shales are dominated by vitrinite macerals (69.1%–96.8%) with minor proportions of liptinite (2.5%–17.55%) and inertinite (0.2%–6.2%). The correlation between maceral composition and S1+S2 indicates that the main contributor to the generation potential is vitrinite. Therefore, the coals and shales of the Pinghu Formation has good hydrocarbon generation potential, which provided a good foundation for coal measure gas accumulation. Furthermore, coal facies models indicates that the Pinghu coal was deposited in limno‐telmatic environment under high water levels, with low tree density (mainly herbaceous) and with low‐moderate nutrient supply. Fluctuating water levels and intermittent flooding during the deposition of peat resulted in the inter‐layering of coal, shale and sandstone, which potentially providing favorable preservation conditions for coal measure gas.

The Development Characteristics and Distribution Predictions of the Paleogene Coal‐measure Source Rock in the Qiongdongnan Basin, Northern South China Sea

“…In the present study, through the analysis of the geological attributes and wave impedance inversions of the typical profiles, it was considered that the results based on model inversion and neural network inversion had good correspondence with the coal bearing sections revealed by drilling processes. That is to say, the low wave impedance corresponded to the coal bearing sections, which coincided with the relatively low density and low velocity of the coal seams (Li Z X et al, 2011;Wang D D et al, 2020). The low density and low velocity of the coal seams had obvious wave impedance differences with the surrounding rock, particularly in the overlying and underlying stratum.…”

“…It was observed that in the depressions where the coal seams were developed, the coal seams had mainly been formed in the delta plains due to the influences of the geological conditions, such as the paleogeography and paleo-structures. The coal seams in the onshore and offshore transitional zones on the delta plains were found to be the best developed, and gradually became thinner and wedged out towards the land and sea, respectively (Li et al, 2010(Li et al, , 2012Wang Y et al, 2020).…”

“…The characteristics of the paleo-structure and paleotopography during coal forming periods play important roles in controlling the coal forming processes. According to the tectonic styles and paleo-topographic features of the Yacheng Formation in the Qiongdongnan Basin during the depositional stages, six typical tectonic-paleotopographical styles have been identified (Wang D D et al, 2020): (1) The Simple Half Graben I style represented by Ya'nan Sag is characterized by consistent sedimentation and settlement center, and is dominated by coal accumulations in a gentle slope belt; (2) The Simple Half Graben II style represented by Songxi Sag has the characteristics of inconsistent sedimentation and settlement center, which is dominated by coal accumulation in the gentle slope belt; (3) The Composite Half Graben I style represented by Songnan Sag is dominated by coal accumulations in an echelon gentle slope zone; (4) The Composite Half Graben II style is represented by the Yabei Sag, in which consistent sedimentation and settlement center can be observed. This style is dominated by coal accumulations in gentle slope belts and secondary uplifts; (5) The Composite Graben Basin style represented by the Songdong Low-Uplift-Baodao Sag region is dominated by coal accumulations in the gentle slope belts of the basin margin; and (6) The Songnan Sag represents the Composite Graben-Half Graben Basin style, which is dominated by coal accumulations in the gentle slope belts of uplift areas.…”

“…The coal measures in the Qiongdongnan Basin were mainly developed in the Yacheng Formation of the Lower Oligocene. Within the sequence stratigraphic framework, it could be seen that the coal seams had been mainly developed during the early stages of the transgressive system tracts and high stand system tracts (Wang et al, 2014;Wang D D et al, 2020). In fact, the coal accumulations in the continental-marine transitional environments (offshore environments) had actually occurred during the processes of the slow rises in sea levels (base levels).…”

Characteristics of Hydrogen‐rich Coals in Southern China: Implications from Organic Geochemistry and Carbon Isotopic Compositions

“…The TOC values range from 56.60%-87.00% of the samples used. Samples are also characterized by high HI (225-555 mg HC/g) and high S 1 + S 2 values (149.37-383.68 mg HC/g), implying good hydrocarbon generation potential (Hakimi et al, 2018;Wang et al, 2020).…”