The primary goal of BPMN is to provide a notation that is readily understandable by all business users, from the business analysts that create the initial drafts of the processes, to the technical developers responsible for implementing the technology that will perform those processes, and finally, to the business people who will manage and monitor those processes.

BPEL is an execution language. There is for example no standard graphical notation in the BPEL standard. The main focus is not on readability, it is on execution.
So BPMN and BPEL aim for different goals. It should come as no surprise that there is gap. There are several resources that describe the mapping, provide translation, or transforming BPMN to BPEL.

Guidelines

Here are some guidelines that help you bridge the BPMN to BPEL gap:

• Be as specific as possible in BPMN diagrams (In Oracle Business Process Analysis Suite you can use e.g. automated activity, notification , and human tasks.).
• Add additional information in BPMN activities. Sure this doesn’t enable automatic transformation, but it does reduce the need for additional design documentation. This will improve documentation consistency, and reduce the required governance.
• Use templates in your BPMN activities. Structure and check completeness of your descriptions.
• Use a Service repository. This will enable both designers and developer to communicate about “the same thing”
• Use a GUI components reposity, for the same reasons as you use a Service repository