It was confirmed from the MZ-1-1 well that the Chaoshan depression had developed two sets of mainly hydrocarbon source rocks in the T₃-J₁ and bathyal—neritic and neritic facies in the J₃. The organic carbon content of the 1800–2000 m section of late Jurassic was 0.18–1.15%, the average was 0.67%, belonging to poor hydrocarbon source rock. The thickness of hydrocarbon source rock in the 2100 m–2400 m section of in the T₃-J₁ was 46.16 m, and the organic carbon content was 1.00–1.48%, with an average of 1.32%, belonging to medium hydrocarbon source rock. These two sets of hydrocarbon source rocks were of organic matter type II₂-III (Yang et al., 2008; Hao et al., 2009).

In the seismic section next to the MZ-1-1 well (Figure 2), the muddy rock layer featured medium-weak amplitude, weak continuous, and low—frequency or blank reflections. From interpretation of the seismic grid, both sets of source rocks thickened outward in various directions from the drilled high Mesozoic (Figure 3). The T₃-J₁ mudstone layer becomes much thick, larger than 1000 m to the northeast region of the eastern sag over where the HD2019-1 well was drilled. The J₃ mudstone layer also thickens, to generally 400–800 m, toward the middle low bulge and the western sag, prevailing in the NE-SW direction.

The mud hydrocarbon source rock of the J₃ mainly develops in the lower layer, its seismic facies characteristics are weak amplitude, medium-high frequency, medium-weak continuous, locally blank reflection, or chaotic reflection configuration (Figure 2), belonging to the bathyal-neritic deposition. This set of mud rock is distributed in the western sag, the middle low bulge, and the southern part of eastern sag, elongating in the NE-SW direction with stable thickness at about 400–800 m (Figure 3).

Organic matter maturity is forwardly modeled based on seismic tying with the MZ-1-1 well where the hydrocarbon source rocks lie mostly below 1700 m. The maturity value, Ro, generally tends to increase sub-linearly with the formation depth from top to bottom (Figure 4), greater than 2.0% below 1700 m, and up to 3.0% at the bottom of the medium-upper Jurassic (about 2400 m). The hydrocarbon source rock has entered the late phase of over mature (Yang et al., 2008). The middle-upper Jurassic source rock reached low maturity to the end of the late Jurassic period. The lower cretaceous reached the late hydrocarbon-generating stage in the middle of the late Cretaceous period (Figure 4).

On the plan view, the thickness of the T₃-J₁ hydrocarbon source rock increased with the formation deposition. Nowadays, except for the northern part of the northern slope and the eastern part of the eastern sag which are still in the immature-low mature stage (Ro<0.7%), the rest of the regions have entered the mature hydrocarbon-generating stage. Most of the western sag, the southern part of the western slope, the southern-central part of the eastern sag, and the middle low bulge central regions are all entering the over mature stage (Ro>2.0%) (Figure 5).

Except in the western slope, J₃ hydrocarbon source rocks are in their immature stage in the middle slope, the northern slope, and the southern part of the middle low bulge; the low maturity-maturity stage is present in the other region. The central and southern part of the eastern sag and the southern part of the western sag enter into the gas generating stage (1.3% < Ro ≤ 2.0%), locally into the over mature stage (Figure 5). Therefore, the Chaoshan depression mainly developed two sets of hydrocarbon source rocks distributed in the NE-SW direction on the plane in T₃-J₁ and J₃. Most of the hydrocarbon source rocks are in their mature to over-mature stage nowadays. The potential for Mesozoic thermogenic gas generation is great.

The Mesozoic in the Chaoshan depression faulted mainly in three strikes: NE, near SN, and near EW (Figure 6). The faults can be recognized as two generations, one during the Mesozoic and another during the Cenozoic Era (Figure 7). The earlier one has offset the Mesozoic sequences, generally with small throws, resulting in fragmented, weak, and chaotic reflections of the Mesozoic sequences. While the young one has more distributed fractures with lasting activity from the Pliocene to the Quaternary, which leads to instability and blank reflections of the young sediments.

Widespread bottom simulating reflectors (BSRs) present the seismic profiles over the northeastern most corner of the Chaoshan depression, named as the NO. I BSR anomaly area, where the 2013 and 2019 drilling surveys were conducted with discoveries of hydrates. As an example, seismic profile C-C' (Figure 7) reveals that BSRs coincidently appeared above the fault-fracture zones, e.g., F₁, F₂, F₃, F₄, F_5, and others. The association of the fault deep into the Mesozoic and fractures within the shallow covering sediments form good channels for the thermogenic gas to migrate vertically upward and diffuse as a hydrate ore body as an expression of the BSRs.

Submarine leakage of hydrocarbon leads to a geochemical anomaly on the seafloor. A quantified geochemical anomaly is calculated as described previously by normalization of five tested items over the sampled region. Its relative variation reflects the varying extent of hydrocarbon leakage.

Figure 6 shows that there are three obvious geochemical anomaly areas in the Chaoshan depression, i.e., in the western slope, the southern part of the western sag, and the junction area of the middle low bulge and the southern part of the eastern sag. They are potential areas of hydrocarbon leakage. Coincidently, these areas are distributed with numerous submarine mud volcanoes (Figure 8) which also usually indicates extensive leakage of oil and gas (Yan et al., 2017).

The overlap map of the geochemical anomalous area and the Mesozoic fault distribution shows a clear correlation between them (Figure 6). The geochemical anomaly is often high over the fault zones.

Most of the mapped faults cut through the two sets of source rocks in the Mesozoic, and break to the Cenozoic. The Cenozoic in this area is thin, generally, less than 1000 m, and does not have the gas source foundation for the formation of thermogenic gas. Therefore, it can be concluded that the hydrate thermogenic gas in this area mainly comes from the Mesozoic.

The thermogenic gas in the lower part of the Chaoshan depression hydrate ore body in Dongsha Waters comes from the Mesozoic (Figure 9). The deep region’s fault-fractures communicate the temperature and pressure stability region of the hydrate and the Mesozoic hydrocarbon source rock, and the deep thermogenic gas is migrated along the transport system to the overlying formation and mixed with the microbial gas, eventually forming hydrate. The fault plays a key role in the process of hydrate accumulation in the region. The Mesozoic fault development area is the main leakage area of thermogenic gas, and the gas source of thermogenic gas in this area is sufficient.

Apart from the northeastern most corner of the Chaoshan depression, the junction area of the middle low bulge and the southern most part of the eastern sag (Figure 6) is also found with a large span of clearly visible BSRs (Figure 10) over where high organic carbon is seen from results of the geochemical survey (Figure 6). Here the Mesozoic sequences develop more completely, with the T₃- J₁, J₃ hydrocarbon source rock entering into a mature–over mature period. Similar fault-fractures, such as F₆, F₈, and F₉, cut deep to Mesozoic, and branch upward as F₇, F_10, and more fractures (Figure 10) connecting with them, forming well passages for the thermogenic gas to migrate upward to the Cenozoic. Moreover, abundant authigenic carbonate nodules and deep-water fauna, e.g., shells, coral, and sponges, are found from dredged samples over some of the mud volcanoes by the area, which implies vigorous leakage of hydrocarbon (Zhong et al., 2020; Yan et al., 2021). Given the gas leakage condition and appearance of BSRs, the junction area of the middle low bulge and the southern most part of the eastern sag can be utilized as a future drilling target.

The oil and gas that leaked from the Mesozoic becomes the main source of thermogenic gas of the natural gas hydrate over the Dongsha Waters. The remaining hydrocarbon still can be traced through future exploration considering their wide distribution.