Fault Evolution and Its Effect on the Sealing Ability of Mudstone Cap Rocks

Abstract

To study the spatial distribution and scale of oil and gas near faults in petroliferous basins, a prediction model is established for the degree of damage that faults in different stages of evolution exert on the sealing ability of mudstone cap rocks by calculating the stages and degree of fault damage to the sealing ability of mudstone cap rocks. This model is applied to the Nanpu 5th Structure and the results show that at survey lines L2 and L8, the F1 Fault destroyed the sealing capacity of mudstone cap rock of the Dongying Formation (E3d2). The undamaged cap rock stage, when the degree of damage was zero, persisted from 23.8 to 16.0 Ma at survey line L2 and from 23.8 to 13.6 Ma at survey line L8. Complete destruction, i.e., where the degree of damage was 100%, at survey lines L2 and L8 occurred from 16.0 to 13.3 Ma and from 13.6 to 13.3 Ma, respectively. The partial destruction stage began 13.3 Ma ago and persists today; the degrees of damage at survey lines L2 and L8 were 89.96% and 82.58%, respectively. This was not conducive to oil and gas accumulation in the reservoir under the mudstone cap rock of E3d2. These results agreed with the current findings of small amounts of oil and gas under the mudstone cap rock of E3d2 at survey line L8 and no oil and gas at survey line L2. This indicates that the model is feasible for predicting the degree of damage to the sealing ability of the mudstone cap rocks by faults at different stages in their evolution.

Evaluation of Lateral Sealing of Faults Based on Porosity: A Case Study of the F1 Fault of the Nanpu No. 5 Structure in the Nanpu Sag

“…It is a normal fault that breaks through the third member of the Dongying Formation (Ed 3 ) at the bottom and near the surface at the top and dips to the northwest with an average dip angle of 25 • (Figure 2b). The results of the fault growth index indicate that the most recent period of this fault activity occurred during the deposition of the lower Ng (about 23.8-13.3 Ma), after which the fault entered a quiescent state, i.e., the fault ceased to be active at 13.3 Ma [22].…”

“…activity occurred during the deposition of the lower Ng (about 23.8-13.3 Ma), after which the fault entered a quiescent state, i.e., the fault ceased to be active at 13.3 Ma [22].…”

“…For burial depths between 3100 m and 3500 m, constructive diagenesis dominates, and porosity is not strongly correlated with these parameters [21]. In addition, the F1 Fault throws in the lower part of the second member of the Dongying Formation (lower Ed 2 ) (burial depth less than 3100 m) are between 20 and 185 m [22], all of which are greater than the thickness of the single sand body in lower Ed 2 (2-13 m) displaced by the F1 Fault and is a fault rock sealing. This study presents an analysis of the mechanism of fault lateral sealing and its influencing factors based on the formation process of fault rocks.…”

Combined Analytic Hierarchy Process and Weighted Interval Method Models for the Geological Evaluation of CO2 Storage in Coal Goaf

“…The higher the clay content in the cap rock, the lower the porosity and permeability of the rock layer, the higher the displacement pressure, and the stronger the sealing ability of the cap rock. Indicators such as the mud ratio or mud-sand ratio are used to evaluate the sealing ability of cap rocks [45,46]. The toughness of rocks also affects the sealing of the cap rock.…”